World War I, which started in July 1914 and ended Nov. 11, 1918, was responsible for more than 17 million casualties, approximately 100,000 of which were troops from the United States. Causes of World War I include mutual defense alliances, imperialism, militarism and nationalism. The assassination of Austria's Archduke Franz Ferdinand on June 28, 1914 was the definitive event that started World War I.
Before World War I began, various countries had treaties in place that required them to join the battle once an ally was attacked. Before World War I started, exiting alliances included Japan and Britain, Russia and France, Germany and Austria-Hungary, and Russia and Serbia. Britain, France and Belgium were also allies.
The desire to take over other global territories, such as those in parts of Asia and Africa, contributed to World War I. Some countries were competing for the resources available in these places. Competition for military power and weapon technology also contributed to World War I. Each country was trying to prove its power and supremacy at the time, and this nationalism played a role.
When Franz Ferdinand was assassinated by a Serbian terrorist group called the Black Hand, Austria-Hungary declared war on Serbia. This set in motion the various factors that culminated in World War I.