In World War I, the Central Powers of Austria-Hungary, Germany, Bulgaria and Turkey, formerly the Ottoman Empire, fought against the Allies. More than 20 countries made up the Allied and Associated Powers, including the United States, France, Great Britain, Russia and Serbia. Japan, Italy and Belgium were also major countries in the Allied forces.
World War I was driven by imperialistic clashes and unstable treaties between European countries. In the years leading up to the war, Serbia and Bulgaria helped force the Ottoman Empire out of the Balkans, while Austria-Hungary and Russia both held major ethnic ties to the Balkans, exacerbating disputes about which nation was entitled to those territories. The French government harbored resentment against Germany following the annexation of French land after the Franco-Prussian War, contributing to France’s decision to form an alliance with Russia.
On July 28, 1914, Austria-Hungary was the first country to declare war after a Serbian nationalist assassinated Austria’s diplomat, Archduke Franz Ferdinand. Austria-Hungary intended to use the conflict to assume control of Serbia, but the government first solidified an alliance with Germany to discourage Russia from retaliating in support of Serbia. Russia began mobilizing on July 25, spurring Germany to declare war on Russia on Aug. 1. Germany's decision to mobilize Belgian forces against Russia's ally, France, violated a neutrality policy, which signaled Great Britain's entry into the war.