World War I was caused because of 4 M.A.I.N causes.One of these causes was Nationalism, which meant that some ethnic groups wanted their own nation.
We want our own nation!
Another M.A.I.N cause of World War I was the Alliance System. This system meant that countries were obligated to help their allies in war.
Hey Russia, wanna help me and declare war against Austria-Hungary?
I guess, yeah.
The single biggest event that led to World War I was the assassination of Archduke Franz Ferdinand. He was assassinated by Serbian nationalists who were upset at Austria-Hungary's annexation of Bosnia-Herzegovina.
Once the war was in full force in Europe, two major alliances were formed; the Triple Alliance and the Triple Entente. The Triple Alliance contained Germany, Austria-Hungary, and Italy. The Triple Entente contained Russia, France, and Great Britain
On April 2, 1917, the United States joined World War I when they declared war on Germany. They did this because of the Zimmerman Telegram and because of German U-Boats sinking civilian ships containing US citizens.
By November 1918, the Allied Powers were triumphant in the war as all Central Powers had surrendered, with Germany being the last to do so. Germany was then forced to sign the Treaty of Versailles, which stated that they had to give up much territory, reduce their army and navy, and pay massive amounts of reparations.