World War 1 started in 1914 in Europe and the U.S. entered the war in 1917. The major causes of the war included militarism, the alliance system between European powers, imperialism, and nationalism. Trench warfare led to massive casualties with new weapons and stalemate on the Western Front. While the U.S. initially remained neutral, American entry was prompted by German attacks on merchant ships, the Zimmerman Telegram, and Germany resuming unrestricted submarine warfare against neutral nations.