World War I began in 1914 after the assassination of Archduke Franz Ferdinand of Austria-Hungary by Serbian nationalists. This caused Austria-Hungary to declare war on Serbia and draw in allied countries on both sides through a series of declarations. New technologies emerged including tanks, planes, machine guns and chemical weapons. Trench warfare led to a stalemate as each side fought from their defensive systems. The US initially remained neutral but later joined the allies in 1917 after German U-boats sank American ships. The war ended on November 11, 1918 with the defeat of the Central Powers. The Treaty of Versailles assigned blame and punishment to Germany.