The Triple Alliance was a military and diplomatic alliance of the German and Austrian-Hungarian empires and Italy, which lasted from 1882 until 1914, when World War I broke out. It was formed to counterbalance the military power of France and Russia, which along with Great Britain would eventually form the opposing Triple Entente.
The Triple Alliance was formed because Italy sought military and diplomatic support against France after losing its African colonies to France in the 1870s. In May 1882, the three countries agreed that Germany and Austria-Hungary would assist Italy if it was attacked by France without provocation. In turn, Italy would assist Germany if attacked by France, though it would remain neutral in case of a war between Austria-Hungary and Russia.
The Triple Alliance was renewed several times after 1882 on slightly different terms. But in 1914, after the assassination in Sarajevo of the Archduke Franz Ferdinand, the heir to the Austro-Hungarian throne, by Serbian nationalists, Austria-Hungary declared war on Russia and Germany declared war against both Russia and France. Italy, asserting that Austria-Hungary was the aggressor, declined to enter the war on behalf of its allies, and the Triple Alliance broke apart. Italy would later enter World War I as an ally of the Triple Entente. Its victory at Vittorio Veneto in 1918 would cause the downfall of the Austro-Hungarian Empire, its former ally.
Germany and Italy, along with the empires of Austria-Hungary, were the countries to make up the Triple Alliance, which was in place from 1882 to 1914.