The alliance system played an important role by leading to World War I, as countries that were in alliances with other countries were obligated to support their allies when war was declared.
In World War I, two major alliances faced each other. On the one hand, the Triple Alliance formed by the Central Powers: the German Empire and Austria-Hungary. Italy, which had been a member of the Triple Alliance together with Germany and Austria-Hungary, did not join the Central Powers, as Austria, against the agreed terms, was the aggressor nation that unleashed the conflict. On the other side was the Triple Entente, formed by the United Kingdom, France and the Russian Empire. Both alliances underwent changes and there were several nations that ended up entering the ranks of one or another side as the war progressed: Italy, the Empire of Japan and the United States joined the Triple Entente, while the Ottoman Empire and the Kingdom of Bulgaria joined the Central Powers.
