<Dl> <Dt> Naval theaters </Dt> </Dl> <Ul> <Li> U-boat </Li> <Li> Atlantic Ocean </Li> <Li> Mediterranean </Li> </Ul> <P> The Western Front was the main theatre of war during the First World War . 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, which changed little except during early 1917 and in 1918 . </P> <P> Between 1915 and 1917 there were several offensives along this front . The attacks employed massive artillery bombardments and massed infantry advances . Entrenchments, machine gun emplacements, barbed wire and artillery repeatedly inflicted severe casualties during attacks and counter-attacks and no significant advances were made . Among the most costly of these offensives were the Battle of Verdun, in 1916, with a combined 700,000 casualties (estimated), the Battle of the Somme, also in 1916, with more than a million casualties (estimated), and the Battle of Passchendaele (Third Battle of Ypres), in 1917, with 487,000 casualties (estimated). </P>

Where did most of the fighting take place during ww1