The country's performance on the battlefield offers a partial explanation for the persistence of the ancient regime in this era of unprecedented strain. To be sure, the war began badly for Austro-Hungarian arms, as its forces were humiliated in operations against both Serbia and Russia. Two efforts to advance into Serbia—one in August 1914 and the other that December — resulted in decisive Serb victories. The empire's armies staggered back in the face of their smaller but highly motivated adversary. In advancing against*the Russians in Galicia, the results were even worse. Austro-Hungarian troops were stopped, then pushed backward. An army of 120,000 men was besieged at Przemysl, which fell in March 1915. The Polish provinces of the empire now rested in enemy hands, Russian troops were advancing on the passes through the Carpathian Mountains, and the plains of Hungary lay open to invasion. The following year brought a decisive reversal of fortune in both areas. With the aid of their powerful ally Germany, Austrian troops took the offensive against Russia at Gorlice-Tamow in May 1915, driving the enemy out of Galicia and deep into Russia's home territory. In October, aided by Bulgaria as well as Germany, Austrian troops overran all of Serbia, then turned to occupy Montenegro as well. The pattern of critical help from Germany available to produce important Austrian victories held for the remainder of the war. In the summer of 1916, Russian troops under General Alexis Brusilov penetrated the Austrian lines and advanced into Volhynia, a part of the Habsburg lands. Once again, German forces were immediately available to lead a successful counteroffensive. When Rumania entered the war and invaded the Habsburg holdings in Transylvania, Germany stepped in still another time to invade Rumania and turn around the direction of the campaign. In August 1917 General Luigi Cadoma's Italian army achieved a victory of sorts on the Isonzo, advancing at heavy cost on the Bainsizza plateau. German troops then provided the edge of the sword for the Central Powers' devastating counterattack on Italy in October 1917, leading to the decisive victory at Caporetto. Without German aid, Austria's military successes—and even its security against Russian invasion—were uncertain. If Russian troops had penetrated to the center of Hungary in the spring and summer of 1915, the consequences would have been profound. But Germany's interest in preserving its only ally and its ready ability to do so staved off this as well as lesser crises. During most of the war, and especially at the time of Caporetto, when the Central Powers broke through the Italian defenses on the Isonzo in October 1917, Austrian authorities had the prestige of military successes to offset the difficulties on the home front.