At the outset of war, the 4th Army, with the 5th Army, formed the center of the German armies on the Western Front, moving through Luxembourg and Belgium in support of the great wheel of the right wing that was to pin down and defeat the French armies. The 4th Army defeated Belgian forces on the frontier, drove the French out of the Ardennes and then encountered the British Expeditionary Force in the "Race to the Sea" at the First Battle of Ypres. The 4th Army faced the British in Flanders for the rest of the war, notably defending in the Battle of Passchendaele (1917), attacking in the 1918 German spring offensive and finally being pushed back in the Hundred Days Offensive from August 1918.