The Church and much of the aristocracy favoured the cause of the Catholic Austrians against the liberal West. But the traditions of the Risorgimento, the prospect of the final unification of the Italian nation, gave the Allied cause a great popular advantage, which the Central Powers could match only by ceding the Italian-speaking territories still in Austrian possession. The Germans brought heavy pressure to bear on their Austrian allies to do this, but Vienna was understandably reluctant. After all, the war was being fought to preserve the Monarchy, not to dismantle it.
The Italians were universally unpopular, besides being the only adversaries the Austrians were confident of being able to defeat. Nevertheless, in May 1915 Vienna reluctantly yielded to German pressure. It was too late: the Italians had signed the secret Treaty of London with the Allies on 26 April. By this they were promised all the Italian-speaking regions south of the Alps, together with wide areas of Slovenia and Dalmatia where the Italians were in a definite minority—to say nothing of a substantial share in Turkish Anatolia where there were no Italians at all.
Italy declared war on 23 May 1915, and her commander-in-chief General Luigi Cadorna spent the next two years launching suicidal attacks in the mountains beyond the Isonzo, losing almost a million men in the process. The Austrian army fought them with an enthusiasm that it had shown on no other front. Arguably, the Italian entry into the war did more for the morale of the Austrian army than the victories it had won, very much as a junior partner to the Germans, over the Serbs and Russians in the course of 1915. Certainly it did little to compensate the Allies for the loss of the Balkans and their defeat at the hands of the Turks.