 Fighting in Eastern Europe began with the first Austro @-@ Hungarian invasion of Serbia in early August 1914 , under the command of Oskar Potiorek . The number of Austro @-@ Hungarian troops assigned to the invasion was far smaller than the 308 @,@ 000 @-@ strong force intended when war was declared . This was because a large portion of the Austro @-@ Hungarian 2nd Army had moved to the Russian Front , reducing the number of troops involved in the initial stages of the invasion to approximately 200 @,@ 000 . On the other hand , the Serbs could muster some 450 @,@ 000 men to oppose the Austro @-@ Hungarians upon full mobilization . The main elements to face the Austro @-@ Hungarians were the 1st , 2nd , 3rd and Užice armies , with a combined strength of approximately 180 @,@ 000 men . The Serbian Army was commanded by Crown Prince Alexander , with the chief of the Serbian general staff , Radomir Putnik , who had commanded Serb forces in the Balkan Wars , as his deputy and de facto military leader . Petar Bojović , Stepa Stepanović , Pavle Jurišić Šturm and Miloš <unk> commanded the 1st , 2nd , 3rd and Užice armies , respectively . 
