Why did the United States enter World War I, and what effect did its entry have on the war?
The American people were overwhelmingly against joining World War I, but ultimately entered the conflict based on two events- the sinking of the Lusitania, and the interception of the Zimmerman Telegram.
German U-boats attacked and sunk the British ocean liner Lusitania in 1915, resulting in the death of 120 American citizens. While this event didn’t lead to a direct declaration of war, it did escalate tensions between the US and Germany and helped sway popular opinion against the Germans. Later the British intercepted a telegram from the Germans (The Zimmerman Telegram) that would ultimately push the United States into a declaration of war. The telegram from Germany urged Mexico and Japan to declare war on the United States to divert her attention from Europe. Once Germany was victorious in Europe it would aide its two new allies in defeating the Americans. Mexico was also promised that it would regain much of the territory it lost to the Americans in the Mexican-American war.
As a result of these events President Truman successfully petitioned Congress for a declaration of war, and officially entered the First World War on April 6, 1917.