Answer: WWI was caused by many things, among them was the assassination of Franz Ferdinand leading to the Austria-Hungary empire declaring war on Serbia which dragged most of Europe into the war due to alliances. The U.S. entered the war due to Germanys war tactics and the Zimmermann note. On April 6, 1917, the U.S. joined its allies--Britain, France, and Russia--to fight in World War I. Under the command of Major General John J. Pershing, more than 2 million U.S. soldiers fought on battlefields in France. Many Americans were not in favor of the U.S. entering the war and wanted to remain neutral.