In 1917, after 3 years of neutrality, United States declared war on Imperial Germany and thereby entered World War I for several reasons. One, unrestricted submarine warfare destroyed American ships. Two, the Zimmerman telegram outraged the United States and gave reason to believe that Germany was planning to wage war directly on the continental US. Three, Russian withdrawal from war with the Bolshevik overthrow of its government dramatically increased the chances of German victory in Europe. .
First, since the very beginning of the war in 1914, United States was a major supplier of food and military supplies to both, the Allied and Central powers. However, due to British navy's successful blockade of German outlets to the ocean, American ships were no longer able to supply Germany with as much success. Instead, United States turned to trading more and more with the Allied powers (Doc. 2). However, this angered Germany, especially since America has declared itself neutral, which was obviously not the case from German perspective. In retaliation, Germany used its U-boats to sink any and all American ships that came close to the coasts of European nations. This infuriated the United States as it caused hundreds of deaths of Americans as well as millions if not billions of dollars lost in profit. In one month prior to the declaration of war, German submarines sank nearly 900,000 tons of American ships.
Second, in January of 1917, the British intelligence intercepted the Zimmerman telegram from Germany to Mexico (Doc. 4), which urged Mexicans to attack the United States for territory, and that Germany will come to the rescue once it is victorious in Europe. This underhanded tactic outraged the American people and made many who were previously neutral strongly opposed to Germany.
Third, a few months before the declaration of war on Germany, a dramatic event took place. Bolsheviks overthrew the Russian and they saw no reason to continue fighting Germany (Doc.