 Complete the following phrase:  Bulgaria declared war on Serbia on 12 October 1915 and joined in the attack by the Austro-Hungarian army under Mackensens army of 250,000 that was already underway. Serbia was conquered in a little more than a month, as the Central Powers, now including Bulgaria, sent in 600,000 troops total. The Serbian army, fighting on two fronts and facing certain defeat, retreated into northern Albania. The Serbs suffered defeat in the Battle of Kosovo. Montenegro covered the Serbian retreat towards the Adriatic coast in the 