Why did the United States enter World War I?
Because Germany attacked U.S. (civilian) ships
During World War I, Germany tried to cut off supplies to its enemies by using submarines to sink ships in the Atlantic. Some of those attacks hit American ships and killed civilians, which turned public opinion and the government toward joining the war.
To support the Allied Powers (England, France, Italy, and Russia)
World War I began in Europe, and the United States first tried to stay neutral. Over time, the U.S. chose to help countries it had strong ties with, especially Britain and France. Joining the war meant fighting alongside these Allied nations against the Central Powers.
To oppose the Central Powers (Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria)
In 1917, the United States joined the war on the side of the Allies to help stop Germany and its partners from gaining more power in Europe. Events like German attacks on ships and attempts to pull other countries into the conflict pushed the U.S. to take action against the Central Powers.