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, and forcing Serbian forces to withdraw to southern Serbia. On 15 October 1915, two Bulgarian armies attacked, overrunning Serbian units, penetrating into the valley of the South Morava river near Vranje up to 22 October 1915. The Bulgarian forces occupied Kumanovo, Štip, and Skopje, and prevented the withdrawal of the Serbian army to the Greek border and Salonika. For a year, the Allies had repeatedly promised to send serious military forces to Serbia, while nothing had materialized. But with Bulgaria's mobilization to its south, the situation for Serbia became desperate. The developments finally forced the French and the British to decide upon sending a small expedition force of two divisions to help Serbia, but even these arrived too late in the Greek port of Salonika to have any impact in the operations. The main reason for the delay was the lack of available Allied forces due to the critical situation in the Western Front. The Entente used Greek neutrality as an excuse, although they could have used the Albanian coast for a rapid deployment of reinforcements and equipment during the first 14 months of the war. (As the Serbian Marshal Putnik had suggested, the Montenegrin army gave adequate cover to the Albanian coast from the north—at a safe distance from any Bulgarian advance in the south in the event of a Bulgarian intervention.) The Entente also delayed due to protracted secret negotiations aiming at bringing Bulgaria into the Allied camp, which event would have alleviated Serbia's need for Franco-British help. In the event the lack of Allied support sealed the fate of the Serbian Army. Against Serbia the Central Powers marshaled the Bulgarian Army, a German Army, and an Austro-Hungarian Army, all under the command of Field Marshal Mackensen. The Germans and Austro-Hungarians began their attack on 7 October with a massive artillery barrage, followed by attacks across the rivers. Then, on the 11th, the Bulgarian Army attacked from two directions, one from the north of Bulgaria towards Niš,