The name given to a period of World War I when, on the Western Front, the two sides were still engaged in mobile warfare. With the German advance stalled at the First Battle of the Marne, the opponents continually attempted to outflank each other through north-eastern France. This process brought the forces back to positions prepared under British Admiralty guidance, on the North Sea coast in Western Belgium. The nature of operations then changed to trench warfare, which is very large scale siege warfare. This produced a continuous front line of trench fortifications more than two hundred miles long, which by the following Spring extended from the coast to the Swiss border.