Out of all the main powers the only country that didn't join the war with their pre-war alliance was Italy, since its alliance with Austria and Germany was only a defensive one, and, as you probably know, Austria was the country that started the war. Another reason why Italy didn't join the war was because the italians still harboured a grudge against the Austrians since they had fought 3 wars against them to gain their independence, so you can understand why they weren't particularly motivated to fight for the central powers. As a matter of fact, thanks to the London treaty, in 1915 they entered the war on The Entente's side, having been promised Tyrol(Trentino in italian), Triest, and Dalmatia(these three regions were in austrian hands and the italians claimed that those territories were rightfully theirs) by the British if they won the war. However, no states had alliances with both sides.

Source: The First World War, Hew Strachan.

Edit 1: Clarification and grammar check