During the first eight months of World War I, the German attack through Belgium into France had been stopped outside Paris by French and British troops at the First Battle of the Marne in early September 1914. The Germans fell back to the Aisne valley, where they dug in. In the First Battle of the Aisne, the Franco–British attacks were repulsed and both sides began digging trenches to economise on manpower and use the surplus to outflank, to the north, their opponents. In the Race to the Sea, the two sides made reciprocal outflanking manoeuvres and after several weeks, during which the British forces were withdrawn from the Aisne and sent north to Flanders, both sides ran out of room. By November, armies had built continuous lines of trenches running from the North Sea to the Swiss frontier.