By the early twentieth century, the Imperial powers of Europe had organized themselves into two competing security structures: the Triple Alliance and the Triple Entente. The Triple Entente linked France, Russia and Britain. The Anglo-Russian Convention of 1907, signed in St. Petersburg, Russia, largely resolved previous shaky relations between Great Britain and Russia. The two imperial powers had long competed in Central Asia and elsewhere. They set aside their differences as the power of Imperial Germany grew. The Triple Alliance was a military coalition between Germany, Austria-Hungary and Italy.
In 1914, Austria-Hungary declared war on Serbia after the assassination of its heir to the throne, Archduke Franz Ferdinand. Russia mobilized in Serbia’s defense. Germany came to the aid of Austria-Hungary, and declared war on Russia and her Triple Entente ally France. To attack France, Germany invaded through Belgium, whose neutrality was guaranteed by Great Britain. Subsequently Great Britain declared war on Germany.
Italy did not enter the war since the Triple Alliance was a defensive alliance, and Italy opined that Germany and Austria-Hungary had taken the offensive. Germany, Austria-Hungary and their allies came to be referred to as the Central Powers. Their opponents came to be referred to as the Allied Powers, or Allies. At the time, the conflict between them came to be called the Great War.