Woodrow Wilson was the US President during WWI. Tensions in Europe led to war in 1914 due to militarism, alliances, imperialism, and nationalism. The assassination of Archduke Franz Ferdinand of Austria-Hungary by Serbian nationalists triggered the war. Though the US remained neutral at first, events like the sinking of passenger ships like the Lusitania by German U-boats and the Zimmerman Telegram eroded American neutrality and led the US to enter the war in 1917 on the side of the Allies.