The prime cause of World War I being the hostility between Serbia and Austria-Hungary, it isn't surprising that some of the earliest fighting took place between Serbia and its powerful neighbour to the north: Austria-Hungary. Serbia held out against Austria-Hungary for more than a year before it was conquered in late 1915. Dalmatia was a strategic region during World War I that both Italy and Serbia intended to seize from Austria-Hungary. Italy entered the war in 1915 upon agreeing to the Treaty of London that guaranteed Italy a substantial portion of Dalmatia.