The commanders of the German and French armies believed that the opening encounters of World War I would decide its fate. Both sides attacked with ruthless intensity during the first clashes of the Great War, known as the Battle of the Frontiers. The Germans saw great success at first forcing the French Army to retreat for almost the entire month of August. The Allies suffered almost 200,000 casualties in the trenches. By early September, the German army had moved so deep into northeastern France that Paris was preparing for a siege when the Allied success at the First Battle of the Marne finally halted the German advance.