Conrad's critics contend that his mistakes led to the disastrous first year of war that crippled Austro-Hungarian military capabilities. For example, in the 1914 Serbian Campaign, led by General Oskar Potiorek, the Serbian Army proved far more effective than Conrad had expected despite the Austro-Hungarian manpower advantage. Undefeated in all major battles, it finally enforced a full-scale retreat of Potiorek's troops by the end of the year. Also the first Austro-Hungarian offensives against Russia were remarkable for their lack of effect, culminating in the lost Battle of Galicia and the disastrous Siege of Przemyśl combined with massive human cost. Conrad was fully responsible for this disaster, for he had committed too many troops in Serbia, leaving severely outnumbered units to resist the Russian advance. Conrad blamed the German allies, who had driven out the Russian Army from East Prussia in the Battle of Tannenberg, for the lack of military support. The most disastrous defeat came in 1916, in the Russian Brusilov Offensive, one of the most lethal battles in world history, whereby the Austro-Hungarian forces under Conrad's command lost more than 600,000 men, and were never again capable of mounting an offensive without German help. The disaster was mostly due to Austrian overconfidence as well as Conrad having sent reinforcements to Italy, ignoring the Russian threat. Most of Austria's victories on the eastern front were possible only in cooperation with the German High Command (OHL), on which the Austro-Hungarian army became increasingly dependent. After his defeats of the first year, Conrad was increasingly sidelined by the Germans on the eastern front.