The Triple Alliance in World War I consisted of Germany, Italy and Austria-Hungary, all of whom had committed to assist each other militarily in case of a war. The purpose of the agreement was "essentially defensive and conservative," but it did not take long for worldwide war to sweep over the continent after its creation. When World War I broke out, the Triple Alliance fought against the Triple Entente.
Germany and Austria-Hungary had some natural reasons for forming an alliance. After all, the two nations shared a common border, and many people in Austria-Hungary also spoke German. The German Empire had only been unified for about 20 years before the Triple Alliance formed and had swiftly become more influential than the aging Austro-Hungarian Empire. Italy joined the Triple Alliance because it was disputing land with Austria-Hungary and thought that an alliance would make it easier to negotiate this disagreement.
France, Great Britain and Russia had formed the Triple Entente, which stood separate from the countries in the Triple Alliance. While having other powers in alliance ostensibly brought military security, it also meant that the alliances had to choose whether to ignore simple acts of aggression by each other or ramp up for war. Because no international arbitration system was in place then, any minor dispute could become a major war, as it did when World War I broke out.