The Western Front of World War I was opened in 1914 after the German Empire's army invaded Belgium and Luxembourg, so it gained military control of important industrial areas of France. The advance of the Empire underwent a dramatic turn after the first battle of the Marne, where the alliance between France and the United Kingdom defeated. Both sides -Aliados and Central Powers- settled in a sinuous line of fortified trenches, which extended from the North Sea to the Swiss border with France. That line remained static for most of the war.