Why did the US enter World War I?

The United States entered World War I primarily because of the increasingly aggressive German submarine campaign in the Atlantic Ocean. German activity was seriously disrupting US exports to Europe, which led American businessmen and industrialists to put pressure on the Wilson Administration to enter the war.

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As World War I began, the United States voiced its intention to remain neutral, a policy which held widespread support among the American people. Yet in April of 1917, the United States would enter the war on the side of the Allies. This raises the following question: what changed in the preceding time to cause such a reversal?

Famously, one of the critical points of contention between the United States and Germany lay in Germany's use of submarine warfare. What you should remember, however, was that the use of submarines was one of the only resources Germany had to counter British naval superiority. However, this tactic, which relied on the use of surprise attacks, resulted in high death tolls and greatly damaged relations between Germany and the United States. The outcry was so great that Germany agreed to offer warnings before launching these attacks, but, under pressure from the British blockade, this policy would not hold, and Germany would later announce its intention to employ unrestricted submarine warfare.

As if this was not enough, there was another critical turning point that would dramatically shape the United States's attitude towards the war. In January of 1917, British intelligence would intercept the Zimmerman telegraph, a secret communication from Germany to Mexico, involving a potential alliance between the two countries should the United States enter the war. These turning points were critical in shaping American intervention, siding with the Allies against the Central Powers.

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The decision by the Wilson Administration to enter World War I on behalf of the Allied Powers was motivated by a mixture of high-ideals and economic self-interest. There can be no doubt that President Wilson genuinely believed that American involvement in the war would help to make the world safe for democracy. At the same time, there were other, less altruistic concerns that he took into consideration before making such an important decision.

The increasing aggressiveness of German submarine activity in the Atlantic was having a damaging impact on American exports to Europe. The Germans attacked and sank a large number of US merchant ships, which inevitably caused consternation among American business interests, who became among the most vocal supporters of the United States' entry into the war. Though he didn't want war and had tried his best to keep the United States neutral in the conflict, Wilson could not ignore the growing clamor for action from the American business community.

Nor, for that matter, could he ignore public opinion. The rising tide of civilian casualties was arguably the biggest single factor behind growing support for US involvement in the war among the American people. The German sinking of the ocean liner Lusitania, which claimed the lives of nearly 1,200 people, had a particularly strong impact upon American public opinion, especially because many of those killed were American citizens.

Before long, support for US entry into the war became too strong for the Wilson Administration to resist, and so the United States formally entered the First World War on April 6, 1917.

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There are two main factors that led the United States to enter World War I after having stayed neutral for so long.

First, there was the fact that the Germans had resumed unlimited submarine warfare.  This had been a point of contention between Germany and the US for most of the war.  The Germans had stopped using the tactic for a time, but started up again in January of 1917.  A number of American ships were sunk in March of that year, leading to the declaration of war in April.   The US felt that this type of warfare was illegal and inhumane.  The government also did not like the fact that it reduced American trade with Britain.

The other immediate cause was the Zimmermann Telegram, which tried to persuade Mexico to enter the war so as to keep the US tied down at home and unable to help the Allies in Europe.  This angered many Americans.  Americans were particularly angered by the fact that the Germans offered to give Mexico land that the US had taken from it in the Mexican-American War. 

These two factors led to the end of US neutrality in this war.

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The primary events that led to the United States declaration of war against Germany were the Zimmerman Telegram and Germany's announced intention to resume unrestricted submarine warfare.

American sentiment had leaned toward the Allies and against the Central powers for some time. Americans felt a common affinity toward the British because of the common language. Also Woodrow Wilson made no attempt to hide his disdain for persons of German ancestry. He once referred to German Americans as "hyphenated Americans."  Sentiment against the Germans was also intensified after the sinking of RMS Lusitania in 1915; however this was NOT the cause of U.S. entry into the war. After that event President Wilson famously commented "there is such a thing as being too proud to fight."

Following the sinking of the Lusitania Germany had issued its Arabic Pledge (sometimes known as the Sussex Pledge) in which the High Command promised it would no longer sink Allied ships without first giving appropriate warning. Several things changed this: The British often few the flags of neutral countries on their ships, and also rammed German U-boats while pretending to allow boarding. Then too the war was at a stalemate, and the Germans had to do something to move the war effort. This led to their notice on January 31, 1917 that Germany would resume unrestricted submarine warfare the next day.

On February 25, 1917, President Wilson received an intercepted German telegram to the German Embassy in Mexico City that offered to Mexico the "lost territory" of Arizona, New Mexico, etc. (land lost by Mexico to the U.S. in the Mexican American War of 1848) if Mexico would declare war on the U.S. The British had intercepted the telegram, and delivered it to the U.S. in an obvious attempt to secure U.S. entry into the war on the Allied side. On March 17, 1917, Germany sank five U.S. merchant vessels, and President Wilson delivered a war message to Congress on April 2. War was declared on April 6, 1917.

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