For the first two months of WWI, both sides engaged in mobile warfare in France and Belgium, but by Christmas 1914 the fighting settled into static trench warfare along the Western Front. An initial German plan to quickly defeat France through Belgium failed at the Battle of the Marne in September. This led to a "Race to the Sea" as both sides tried to outflank each other, ultimately resulting in a line of opposing trenches from the North Sea to the Swiss border. The Battle of Verdun, which began in February 1916 as a German effort to bleed France dry defending the symbolic fortress city, became the longest battle of WWI, lasting until December 1916 with enormous casualties on both sides. In an effort to relieve pressure on