When World War I began, the United States was neutral. However, the United States eventually joined the war on the side of the Allies. One reason this occurred was the German use of unrestricted submarine warfare on American merchant ships. As a neutral nation, the United States had the right to trade with any country, including countries at war. The Germans believed the United States was supplying the Allied nations with war materials and began to sink American ships without warning. With the Sussex Pledge in 1916, Germany agreed to stop sinking American ships. However, early in 1917, Germany broke the Sussex Pledge and again started to sink American ships without warning. This ultimately led to the United States joining World War I.
The Americans were also unhappy with Germany when news of the Zimmermann Telegram became public. The Germans wanted Mexico to attack the United States in order to create a two-front war for the Americans. In return, if Germany won the war, Germany offered to return some of the lands that Mexico lost to the United States in the Mexican-American War. Many Americans were furious with Germany as a result of this action.
There were also more similarities between the United States and Great Britain than between the United States and Germany. The United States and Great Britain had the same language, shared a similar (but not identical) form of government, had a good trade relationship, and had a similar legal system. While these similarities alone would not have led the United States into the war, many Americans sympathized more with Great Britain than with Germany. When the actions described above occurred, they fueled American support for the Allies.