World War I was instigated by the Serbian terrorist-led assassination of Austria-Hungary's Archduke Franz Ferdinand. The country declared war on Serbia one month later on July 28, 2014.
Bound by ties and treaties, fellow Central Powers of Bulgaria, Germany and the Ottoman Empire joined Austria-Hungary in their defense. Serbia was assisted by Allied Powers of Belgium, France, Great Britain, Greece, Italy, Montenegro, Portugal, Romania, Russia and, eventually, the United States.
While the United States was neutral at the onset of the war, it joined the Allied Powers when German submarine warfare threatened American commercial shipping. The United States officially declared war on Germany on April 6, 1917, following the interception of the Zimmermann Telegram and the sinking of three U.S. ships.
On Nov. 11, 1918, Germany reluctantly became the last of the Central Powers to agree to armistice terms sought demanded the Allied forces, ending the war. In signing the Treaty of Versailles the following year, the nation was forced to acknowledge guilt for the war, pay other countries for the damages caused and reduce the size of its military. Germany was also forced to return territories claimed during the war to Belgium, Denmark, France and Poland.
The world's first global conflict marked the introduction of modern warfare, resulting in an estimated 8 to 9 million military deaths worldwide among the countries involved. The 1916 Battle of Verdun in France was the deadliest, resulting in nearly 1 million casualties.