The Western Front was the main theatre of war during World War I. Following the outbreak of war in August 1914, the German Army opened the Western Front by invading Luxembourg and Belgium, then gaining military control of important industrial regions in France. The tide of the advance was dramatically turned with the Battle of the Marne. Following the race to the sea, both sides dug in along a meandering line of fortified trenches, stretching from the North Sea to the Swiss frontier with France. This line remained essentially unchanged for most of the war.