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.
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).
To break the deadlock of trench warfare on the Western Front, both sides tried new military technology, including poison gas, aircraft and tanks. The adoption of better tactics and the cumulative weakening of the armies in the west led to the return of mobility in 1918. The German Spring Offensive of 1918 was made possible by the Treaty of Brest-Litovsk that ended the war of the Central Powers against Russia and Romania on the Eastern Front. Using short, intense "hurricane" bombardments and infiltration tactics, the German armies moved nearly 100 kilometres (60 miles) to the west, the deepest advance by either side since 1914, but the result was indecisive.
The inexorable advance of the Allied armies during the second half of 1918 caused a sudden collapse of the German armies and persuaded the German commanders that defeat was inevitable. The German government surrendered in the Armistice of 11 November 1918, and the terms of peace were settled by the Treaty of Versailles in 1919.