Why did the United States enter World War I?

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The most common explanation for the US entry into WWI emphasizes that the US entered the war because of Germany’s policy of unlimited submarine warfare.  The US disapproved of Germany’s submarine warfare because it seemed cruel and because it was contrary to international laws that were supposed to guarantee freedom of the seas.  The US had been outraged, for example, when a German submarine sank the passenger liner Lusitania with over 1,000 civilians (among who were more than 100 Americans) onboard.  The Germans stopped engaging in this sort of tactic for a while in response to American criticism.

When Germany resumed unrestricted submarine warfare in 1917, US public opinion turned more strongly against the Germans.  This hostility was exacerbated by the revelation of the Zimmermann Telegram, in which the Germans tried to get Mexico to enter the war on the German side in exchange for which the Germans would defeat the US and give Mexico back the land the US had taken from it in the Mexican-American war.  This outraged Americans who felt that it was wrong for Germany to try to incite the US’s neighbor to go to war and who felt that it was wrong for Germany to promise to take away US territory.

Historians, then, tend to say that the US entered WWI because of its opposition to unrestricted submarine warfare and because of the Zimmermann Telegram.

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Explain what caused the United States to enter World War I. 

When World War I broke out, the Wilson Administration's policy was to maintain a strict neutrality. This proved difficult to do, however, as most (but by no means all) Americans favored the Allied Powers, especially Great Britain. Moreover, the United States insisted on maintaining trade relations with all belligerent powers, and since Great Britain had imposed a blockade on German ports, this meant in effect that the United States only traded with the Allied Powers.

Germany responded by launching submarine attacks on Allied shipping, which often resulted in American losses of life and property, most famously when a U-boat torpedoed the Lusitania, a luxury liner that was surreptitiously carrying munitions, killing 128 American passengers. In response to US protests, the German government agreed to restrict its U-boats to military targets in early 1916. Yet one year later, facing starvation due to the British blockade, the German high command chose to resume unrestricted submarine warfare in a desperate bid to weaken the British economy.

This decision was accompanied by a (possibly forged) telegram to the German minister in Mexico, instructing him to offer terms for an alliance between Germany and that nation. These two events, combined with the sinking of a handful of American ships in March of 1917, caused Wilson to ask Congress for a declaration of war in April of that same year.

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