1. The assassination of Archduke Franz Ferdinand led to escalating tensions between nations and their alliances that resulted in World War I. 2. Germany invaded Belgium and France in accordance with the Schlieffen Plan but was stopped at the First Battle of the Marne, resulting in trench warfare on the Western Front. 3. The United States entered the war in 1917 on the side of the Allies after Germany resumed unrestricted submarine warfare, helping ensure an Allied victory.