After the victory of the Serbian army in the Battle of Kolubara in December 1914, the Serbian front saw a lull until the early autumn of 1915. Under the command of Field Marshal August von Mackensen, the Austro-Hungarian Balkan Army, the German 11th Army and river flotillas on the Danube and the Sava began an offensive on 6 October 1915, the largest offensive against Serbia. By September 1915, despite the extreme sacrifice of the Serbian army, the Austro-Hungarian Balkan Army, having crossed the rivers Sava and Drina and the German 11th Army after crossing the Danube, occupied Belgrade, Smederevo, Požarevac and Golubac, creating a wide bridgehead south of the Sava and Danube rivers. This, combined with the successful Bulgarian offensives at Morava, Ovče Pole and Kosovo forced Serbian forces to withdraw to neutral Greece. France and Britain to transferred troops from the Gallipoli Campaign to Greek Macedonia. The Macedonian front was thus established in an effort to support the remnants of the Serbian army to conquer Vardar Macedonia.