It was actually among four countries; Austria-Hungary, Germany, Ottoman Empire (Turkey), and Bulgaria started as an alliance. Germany lead the Central Power group.
Among the three countries, Germany had the greatest military advantage. It is also the militarist war designer and strategist that led their alliance.
Austria- Hungary unification and involvement during World War I started after the assassination of their King Archduke Ferdinand. Austria and Hungary believed that it was a Serbia assassination plot.
The Ottoman Empire signed the alliance with Germany because of its good economic relationship.
Bulgaria who came in last in the Alliance. It claimed that Serbia took a part of its land that made it get to war.
(got this from another question but I hope it helped)
