In 1917 The United States declared war on the Central Powers and sent troops to fight after first wanting to remain neutral. Why did this happen?
Americans did not want to take sides in World War I for two reasons. First, they felt the war did not affect the United States. Second, while the US was an English-speaking nation, there were many people of German descent and Irish descent who were not eager to side with the Triple Entente. Therefore, the US stayed out of the war as long as it could.
But a number of events made this impossible to sustain. The main issue that brought the US into the war was unlimited submarine warfare on the part of the Germans. The Germans wanted to blockade England to deny it access to the supplies it needed. Some of these supplies, of course, were coming from the United States and/or were carried in American ships. When the Germans sank some of these ships and American lives were lost, American opinion turned against the Germans. The Germans stopped engaging in unlimited submarine warfare for a time but then resumed. This was the main factor that pulled the US into the war. Another important factor was the Zimmermann Telegram. This was a telegram in which the Germans tried to entice Mexico to declare war against the US. If it did so and the Central Powers won the war, Germany offered to restore to Mexico some of the land it had lost in the Mexican-American War. This enraged Americans.
For these reasons, the US ended its neutrality and entered the war in 1917.