 The first five months of World War I had seen an initial German attack through Belgium into France , which had been repulsed outside Paris by French and British troops at the Battle of the Marne in early September 1914 . The Germans fell back to the Aisne valley , where they prepared defensive positions . In the subsequent Battle of the Aisne , the Allied forces were unable to push through the German line , and the fighting quickly degenerated into a stalemate ; neither side was willing to give ground , and both started to develop fortified systems of trenches . To the north , on the right of the German army , there had been no defined front line , and both sides quickly began to try to use this gap to outflank one another . In the ensuing " Race to the Sea " , the two sides repeatedly clashed , each trying to push forward and threaten the end of the other 's line . After several months of fighting , during which the British forces were withdrawn from the Aisne and sent north into Flanders , the northern flank had developed into a similar stalemate . By November , there was a continuous front line running from the North Sea to the Swiss frontier , occupied on both sides by armies in prepared defensive positions . 
