In what possible way was it a stalemate? The Hindenburg Line was broken open in a single day by the British Fourth Army on 29 September 1918. That very same day, the Imperial German Army admitted to the Kaiser and the Chancellor that the military situation was beyond recovery and advised seeking an armistice. By the time of the Battle of the Sambre on 4 November, the British Second Army had advanced over fifty miles from the Immortal Salient of Ypres, with ten thousand German prisoners taken in that one battle. Contrary to what the likes of Lundendorff and Hindenburg would later claim, the German Army was utterly defeated in the field. Germany was convulsed by revolution caused by the privation inflicted by the Royal Navy's blockade. There was no question of negotiating an armistice, it was practically an unconditional surrender. The Germans were required to completely withdraw from France and Belgium, almost totally demilitarise, and accept Allied zones of occupation in the Rhineland while the treaty to formally end the war was discussed. They accepted all those demands, because they had lost on the battlefield, and they had lost at home. So in what possible way was it a stalemate?

Sources:

Gordon Corrigan, Mud, Blood and Poppycock

Saul David, 100 Days to Victory

John Keegan, The First World War

Simkins, Jukes and Hickey, The First World War: The War to End All Wars

Hew Strachan, The First World War