Reasons for American Entry Into WWII

Updated on February 6, 2019
JDF78 profile image

I've always been fascinated with history, particularly the history of World War II.

President Franklin Roosevelt signing a declaration of war against Japan on December 8th, 1941. The declaration officially brought the United States into World War II.
President Franklin Roosevelt signing a declaration of war against Japan on December 8th, 1941. The declaration officially brought the United States into World War II. | Source

When Did the U.S. Enter WWII?

While World War II had been raging in Europe since 1939, the United States did not intervene until after Japanese planes bombed Pearl Harbor in 1941. As Japan had an alliance with Germany and Italy, both nations declared war on the United States on December 11th, 1941, four days after the Pearl Harbor attack. This brought the US officially into the war, though there are other reasons why the US entered the war beyond the Japanese attack on Pearl Harbor.

Why Did the U.S. Enter WWII?

World War II was a terrible event that will be remembered as one of the darkest chapters in human history. With estimates of the dead ranging from 60 to 80 million, it is unthinkable to imagine that this event was allowed to fester and erupt as it did. Many in the United States simply figured the problems of Europe would be contained to that continent. However, a new enemy brought the war to our shores.

When the war began, the United States had entered a period of isolationism. Americans viewed the conflict as Europe’s problem and wished to keep it that way. However, as the situation in Europe grew increasingly dire, the United States began to slowly edge toward war.

The breaking point, of course, was the sudden attack by the Japanese at Pearl Harbor. However, in retrospect, the attack may not have been such a sudden and unforeseen event. Tensions between the United States and Japan had been consistently escalating for several years before the attack. However, it was this act of violence that officially caused war to break out.

Reasons for the United States Entering WWII

  1. The Japanese Attack on Pearl Harbor
  2. Japanese Control of China and Asia
  3. Germany's Aggression and Unrestricted Submarine Warfare Sinking U.S. Ships
  4. Fear of German Expansion and Invasion

The USS Arizona sinking in Pearl Harbor after the Japanese attack.
The USS Arizona sinking in Pearl Harbor after the Japanese attack. | Source

1. The Attack on Pearl Harbor

In their bid for regional domination, Japan had begun a campaign of seizing territories around them to get more natural resources and not have to rely on getting supplies from the U.S. Their plan would involve taking over the oil-rich Dutch East-Indies and British Malaya, thereby giving the nation an infinite supply of natural resources. However, the Japanese knew that the United States and the West would not let this happen without a fight. The Japanese surmised that to even the odds they would have to reduce the capabilities of the U.S. Navy in the Pacific region. Therefore, the plot to attack Pearl Harbor was developed.

The attack on Pearl Harbor on December 7, 1941, was initially a striking blow to the United States’ ability to wage war in the Pacific. The morning after the attack, the United States declared war on Japan. However, Japan had scored a tactical victory and was able to mop up what was remaining of the United States military in the Pacific, promptly taking over Guam, the Philippines, British Malaya, and many other small islands and territories in short succession.

Japanese Control of China and the Surrounding Areas

This map shows the extent of Japanese military campaigns in the Pacific.
This map shows the extent of Japanese military campaigns in the Pacific.

2. Japanese Control of China and Empire-Building

While the United States was suffering through the economic meltdown of the Great Depression, Japan was fervently digging its way out of a financial crisis of its own. The Japanese decided that their best hope for survival hinged in their ability to expand militarily. In following this philosophy, the Japanese attacked and occupied the southern region of Manchuria in the fall of 1931. The purpose of this attack was to give Japan a territory rich in raw materials on the mainland. The only problem was that Manchuria was already under the control of China and was an area of strategic importance to the USSR.

Although the United States distrusted the USSR because of the recent communist takeover, the two nations maintained relatively sociable relations at the time. Irritated that the Japanese had moved into their backyard, the USSR began to strongly criticize the Japanese and began to posture militarily in the northern Manchuria region. Due to the United States' relatively amicable relations with both the USSR and China, it too began to publicly criticize the Japanese for their increasing aggression.

The United States cautioned Japan against further aggressive actions by threatening to cut off shipments of raw materials to the nation. This was an especially risky situation for Japan, its only source of oil and metal came from the United States, while their primary source of rubber came from the British territories in Malaya. Therefore, it would seem that the nation would have to tread lightly to avoid angering the West. Or would it?

In a surprising act of defiance, Japan promptly separated from the League of Nations, which was the precursor to the United Nations. Tensions continued to mount in the region for several years until 1937 when Japan entered full-scale military combat with the economically depressed nation of China. This conflict became known as the Second Sino-Japanese War, which would later be singled out as the starting point of World War II in the Pacific Theater.

In the fall of 1940, Japan met with Nazi Germany and fascist-controlled Italy to create an alliance known as the Tripartite Pact. Under this agreement, these three nations agreed to work with and support each other in their respective countries’ efforts to create a new world order.

Japanese, German, and Italian leaders celebrate the signing of the Tripartite Pact.
Japanese, German, and Italian leaders celebrate the signing of the Tripartite Pact.

U.S Involvement and the Lend-Lease Act

The United States responded by beginning to funnel money and equipment to the embattled Chinese. This aid, covered under the Lend-Lease Act, was a tool used by the United States to provide aid to friends and allies without having to directly become involved in the conflict. Also receiving aid from the United States were Great Britain and the USSR as those nations struggled to fend off the growing Nazi threat in Europe.

This move further agitated the Japanese and began to turn an already very uneasy relationship into an outright hostile one. Although the Japanese had angered the West and isolated itself from the world, the nation continued its aggressive tactics. In line with this militaristic movement, the nation then attempted a takeover of French Indo-China. The West had officially had enough of Japan’s belligerence and promptly cut off supplies of natural resources to the regime. This led to Japan devising the plan to attack Pearl Harbor and cripple the U.S. Pacific fleet.

Germany and Italy Declare War on the U.S.

True to the agreement of the Tripartite Pact, Germany and Italy declared war on the United States on December 11, 1941. Interestingly, the United States was slow to respond to Japan militarily. Instead, President Roosevelt and British Prime Minister Winston Churchill devised a strategy to vanquish the European threat before fully concentrating on defeating Japan; this became known as the Europe First or Germany First strategy. Although Japan was a serious threat, the Allied leaders determined that they could be contained to the Pacific region; after all the Japanese were bogged down with the war in China. While conversely, the Nazi’s had wreaked havoc and destruction across all of Europe and even portions of Africa.

Therefore in a surprising twist, the United States went from being attacked by the Japanese to attacking the Axis powers in Europe in only a matter of days. This has led some to speculate that President Roosevelt somehow orchestrated or welcomed the attack on Pearl Harbor as a way to allow the United States to slip into the war in Europe. However, there were many signs that the United States’ entry into the war in Europe may have been inevitable regardless of the events at Pearl Harbor.

3. Unrestricted Submarine Warfare and Growing Tensions With Germany

Much like it had done in WWI, Germany eventually lifted its ban on unrestricted submarine warfare and began attacking merchant ships that were accompanying British vessels in the Atlantic Ocean. As the United States had begun giving more and more resources to their French and British allies, the English navy would help protect American ships that were transporting supplies. This greatly angered Germany, who knew that the United States was using its neutrality as an advantage to help their British allies.

Eventually, Germany resumed unrestricted submarine warfare and began attacking merchant ships and U.S. ships, meaning that was only a matter of time before America would enter the war, especially considering their contentious relationship with Germany.

Continued Tension With Germany

Tensions between the United States and Germany had continued since the end of World War I. Nazi party leader Adolf Hitler saw the United States as a weak yet overbearing nation that consistently meddled in the affairs of other nations. Hitler saw the USA as an ideological enemy, racially mixed and therefore inferior. He also assumed America would be busy fighting Japan while Germany concentrated on taking over the USSR. With the threat from the USSR vanquished he would then be free to finish off Britain with little interference from the Americans.

Much of Hitler’s rationale for carrying out his war and anti-Semitism came about due to the repercussions of World War I. Austrian by birth, Hitler had served in the German army in World War I. According to reports, he was utterly devastated when Germany was defeated. So much in fact, that he never fully recovered from the embarrassment. In turn, he began to blame Jews, Communism, and Western meddling for the despair that had befallen Germany. Determined to see the nation restored to her previous glory, Hitler soon joined a growing movement called the National Socialist German Workers Party or Nazi Party.

Adolf Hitler
Adolf Hitler | Source

The party came to see the Treaty of Versailles, the agreement that ended World War I, as responsible for the destruction of German pride and success. The Treaty of Versailles had been predominantly constructed by the Allied nations of the English, French, and the United States. The treaty was designed in such a matter that Germany would be punished severely for its role in World War I, yet it should be lenient enough to allow Germany to resist the communist movement that was underway in the USSR.

Under the agreement, Germany was allowed to possess no submarines, no military aircraft, and only a few naval vessels. The nation was also forbidden to once again unite with Austria, or create any more secret treaties. And to top it off, Germany had to make reparation payments to the nations that it had attacked. President Woodrow Wilson had little interest in harshly punishing Germany. Instead, he championed the aim of creating a treaty that would allow Europe to handle any future conflicts without the help of the United States.

The Treaty of Versailles.
The Treaty of Versailles.

U.S. Isolationism and the Neutrality Acts

This mentality began to permeate the United States and culminated in the creation of the Neutrality Acts in the 1930s. In essence, the Neutrality Acts tied the hands of the United States to help its allies by refusing to sell resources or loaning cash to any war combatants. However, the Neutrality Acts did have some shortcomings which allowed many American businesses to continue supplying resources to whomever they pleased. Nevertheless, as far as the United States government was concerned the country was to keep the sole focus on itself and remain isolationist.

While the Treaty of Versailles had been developed to remain somewhat lenient, Germans saw it as anything but. Instead, it was viewed as a punishment that was meant to embarrass Germany that was sucking the lifeblood from their nation.

The loss of vital industrial territory would be a severe blow to any attempts by Germany to rebuild her economy. Coal from the Saar and Upper Silesia, in particular, was a vital economic loss. Combined with the financial penalties linked to reparations, it seemed clear to Germany that the Allies wanted nothing else but to bankrupt her.

The Crumbling German Economy

This sentiment proved to be true as Germany's unemployment rate and inflation began to paralyze the nation's economy. The United States tried to step in and help by introducing the Young Plan in 1929. However, this arrangement soured when the United States entered the Great Depression later that same year. The economic instability in the United States created a massive wave of financial collapse around the world, including Germany. In 1933, Hitler and the Nazi Party were able to take control of the German government and immediately set about undoing the Treaty of Versailles. Hitler immediately set about rebuilding Germany’s military forces, to levels that far exceeded the maximum outlined in the Treaty of Versailles. The nation also began to rebuild prohibited military equipment such as military aircraft, tanks, naval vessels, and artillery.

German Aggression

In 1936, the German military invaded and occupied an area called the Rhineland that had been set aside as a demilitarized zone by the Treaty of Versailles. As Hitler had predicted, none of the Allied nations responded to this flagrant breach of the treaty. This lack of a response only served to embolden the Nazi's. Knowing that violating the Treaty of Versailles would have virtually no repercussions, Germany began swallowing up Europe through trickery, lies, and force. When Germany invaded Poland, President Roosevelt was finally able to persuade Congress to allow the exchange of war materials to our allies on a cash and carry basis only.

U.S. Intervention in Europe

However, it wasn’t until Europe was on the brink of total collapse that the United States began to seriously intervene. In July of 1940, France surrendered to Germany, leaving only England and the USSR to fight the Nazi onslaught in Europe. Hitler knew that the only hopes for England's survival depended on aid from the United States and the USSR. However, he also knew that he would not be able to wage a successful campaign against the American's on their home soil. Therefore, he decided to postpone his attack on Britain and instead focus on eliminating the USSR. Germany believed that this would create such a size disparity that it would be impossible for the United States to wage any type of campaign in Europe.

Due in part to increasingly hostile run-ins with Nazi warships and submarines, such as the attacks on the SS Robin Moore and the USS Rueben James, President Roosevelt finally convinced Congress to break away from the Neutrality Act and activate the Lend-Lease Act. The United States then began sending massive amounts of military equipment, and financial support to both Britain and Russia, instituted a military draft, and expanded its naval boundaries. The United States also agreed to supply Britain with 50 naval destroyers in exchange for several military bases in the Atlantic and Pacific.

To protect the shipments of these goods provided under the Lend-Lease Act, the United States Navy then began to escort Allied shipping convoys across the Atlantic. Hitler began to sense that President Roosevelt had been increasing naval activity in the area simply to create an incident that the United States could claim as an act of war. Therefore, on the eve of Germany’s invasion of the USSR, he ordered his naval forces in the Atlantic to not fire on American ships under any circumstance.

4. Fear of German Supremacy

However, the USSR proved to be a much tougher opponent than predicted and was able to slow the Nazi advancement. This bought some time and allowed the United States and England to further fine-tune their strategy. In the fall of 1941, President Roosevelt and Winston Churchill met and established the Atlantic Charter. The agreement set forth the goals for the postwar years, such as freedom of the seas, access to raw materials, global cooperation, and self-government. Most importantly, it called openly for “the final destruction of the Nazi tyranny.”

Indeed, the United States was well on its way to war regardless of its isolationist attitude. It was something that President Roosevelt had come to realize over the years as the Nazi’s continued their path of destruction. In a speech given by the president at the commencement address of the University of Virginia in 1940, he indicated that the United States would have to intervene at some point. He explained that the United States’ view that an isolationist mentality could protect us was delusional, and the evil spreading across Europe would inevitably reach our shores.

Further driving the United States away from its isolationist policies and mindset was the recent advent of motion pictures and radio. These new technologies allowed the American people to see and hear unfolding events in distant places as they never had been able to before. Movie theaters showed the atrocities occurring in Europe and Asia to the masses and radio described the harrowing events in detail. Even before the United States entered the war, the American people began to dislike Hitler, and there was a growing sentiment that he had to be stopped.

Although the American people and Roosevelt were beginning to sense an inevitable intervention, the president knew that he would not be able to convince Congress to declare war until the events directly affected the United States. After all, Congress had only recently allowed for the enactment of the Lend-Lease Act. It was also the same Congress that had sat idly by and let the world descend into chaos. Therefore, convincing them to take action was going to be an uphill battle, to say the least.

It wasn’t until the attack on Pearl Harbor that President Roosevelt was finally able to convince Congress to allow an American response. An interesting side-note, there was still one Congress member that voted against America entering the war. Jeannette Rankin of Montana refused to allow an American response to the attack on Pearl Harbor. Nevertheless, the remaining members of Congress relented and finally allowed for American intervention in the war.

Hitler announces the declaration of war against the United States to the Reichstag.
Hitler announces the declaration of war against the United States to the Reichstag. | Source

Works Cited

Trueman. (2015, March 17). The Treaty of Versailles - History Learning Site Treaty of Versailles 1919. Retrieved on February 5th, 2019.

When Did America Enter WW2? (2018, July 06). Retrieved on February 5th, 2019.

World War II (1939-1945). (n.d.). Retrieved on February 5th, 2019.

Questions & Answers

    © 2011 Justin Ives


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      • profile image


        4 weeks ago

        this is very usefull and easy to understand. thank you and keep it up!

      • profile image


        7 weeks ago

        Nice thanks bro and for you

      • profile image


        3 months ago

        hitler shot himself in the foot (when he invaded the ussr)and the head (when he poisined himself)so hes dunb

      • profile image


        5 months ago

        thank you for doing this and writing it because i had to wright it down for social studies like fr i'm in class right now.and i love my name dont you?

      • profile image


        5 months ago

        Thank you for the information

      • profile image


        14 months ago

        Please don't use England and Britain interchangeably. Thanks.

      • profile image

        Helpfull Man 

        17 months ago

        Thanks!!!! Helpfull

      • profile image


        18 months ago

        who is the author of this article. my daughter is trying to site a source but needs the author?

      • profile image

        Understand This Is History We're Talking About 

        18 months ago

        This history or website is a very important website. It tells us about the war. Not only that but this is some of the stuff that goes with the "The Dairy Of Anne frank" and tells us something went down back of the day. So.. you gotta respect what the person made this website and what its telling you.. Be thankful that you guys didn't have to deal with it..

      • profile image


        18 months ago

        Was very good

      • profile image

        Bo ob 

        18 months ago

        i lov

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        18 months ago

        screw germany!

      • profile image


        19 months ago


      • profile image

        alley cat 

        20 months ago

        Love it

      • profile image

        Shim Slady 

        20 months ago

        there issa lotta words

      • profile image

        im dead 

        20 months ago

        thanks this is good dude

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        20 months ago

        this is really helpful thank you!

      • profile image

        Deez Nuts 

        20 months ago

        This was some really useful information!!!

      • profile image

        fart nuget 

        20 months ago

        thank you it was helpfull

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        20 months ago

        that writing is great and helpful.. thank you very much :)

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        21 months ago

        i needed this thank you

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        21 months ago

        This was very helpful

        I like it

      • profile image

        Biggie C 

        21 months ago

        this stuff is some smart stuff

      • profile image

        Joe Wyze 

        23 months ago

        This actually is a very well written item. I learned very much from this.

      • profile image

        Joe Wyze 

        23 months ago

        This actually is a very well written item. I learned very much from this.

      • profile image

        D Sengupta 

        2 years ago

        Really a informative read... Got lot of info on this...

      • profile image

        Daniel Marais 

        2 years ago

        It was an absolute pleasure reading your article. This insight will really contribute to a lot towards my history essay and I really appreciate the help.

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        2 years ago

        it reallly help me thank u

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        2 years ago

        thxs for the info

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        4 years ago

        this was helpful

      • profile image

        Sarah Goodwich 

        6 years ago


        you say "Hitler was stupid enough to attack the Soviets," as if they had no global ambitions before that, and somehow the war made Stalin decide to conquer the world.

        But in in reality Hitler was simply reading the future correctly, that Stalin was coming after Germany first; and he was simply engaged in a preemptive strike. But that's politically incorrect, so it's banned from vict0r-written-history books.

        In reality, it was Britain that was stupid to declare war on Germany, rather than let the two fight it out; but FDR obviously made secret promises of Lend-Lease in order to spur Britain into being his errand-boy, so he could count on German counter-attacks to keep his promise to keep the US going to war, by doing everything in his power to make the war come to the US instead.

        And as victor-written history shows, it was a rousing success: as well as proof of the motto "absolute power corrupts absolutely...." and also "the first casualty of war, is truth."

      • profile image


        6 years ago

        Truthwill Out, don't forget the Flying Tigers in Japan, along with massive unilateral defense-treaties made AGAINST the American interest.

        Also don't forget that Lend-lease STRIPPED the Pacific Fleet to HALF its original size, which FDR deliberately did in order to BAIT Japan into striking after he attacked them in China, forcing Kimmel to bottle the fleet up in Pearl Harbor due to dwindled numbers making them prey to Japanese mines and midget-subs.

        FDR was the puppet-master of WW2, and the war was his Muppet Show.

      • pconoly profile image

        Pamela K Conoly 

        7 years ago from Fort Worth,Tx.

        Wow! My Dad told me about a lot of this stuff. He was a medic and would have to go and get, sometimes badly injured, soldiers in the midst of all the fighting.But He never got shot or even lost a limb. The stories he use to tell us! It was an amazing war. Great article!

      • profile image


        7 years ago


        Really interesting article!

        Just one thing really, in June 1940 when France surrendered, the USSR was still on Germany's side. So Britain was completely on its own against a hostile or neutral Europe for about a year before Hitler was stupid enough to attack the Soviets.

      • profile image

        Truthwill Out 

        7 years ago

        Cute Whitewash of FDR's consistently brazen warmongering. It is a matter of record that he unilaterally initiated many acts of war against the Axis powers for YEARS while simultaneously reassuring the American people he was opposed to US involvement in any war. You mention Lend Lease yourself as if it was diapers and formula FDR was providing. Providing the Chinese & the Soviets with critical military armaments arguably changed the outcome of the war. Not to mention the equipment was killing Armies of Germans & Japanese soldiers. It was an act of war & FDR knew exactly what he was doing. Any FDR apologist who claims his motivation was altruistic is proven a liar by the historic record. During the late war FDR was so concerned with freedom he gave "Uncle Joe" Poland & all of Eastern Europe. He also forgot to pay any attention to China once he had his war. And he managed to gain control of the Empire that a few years earlier was British. If that wasn't enough Great Britain was left bankrupted by ww2. Only decent thing to happen was FDR's demise.

      • phdast7 profile image

        Theresa Ast 

        7 years ago from Atlanta, Georgia

        Very detailed and thorough Hub. Great pictures and graphics as well. I look forward to reading more of your Hubs.


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