Following the outbreak of World War I in 1914, the German Army opened the Western Front by first 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.
Between 1915 and 1917 there were several major offensives along this Western Front. The attacks employed massive artillery bombardments and massed infantry advances. However, a combination of entrenchments, machine gun nests, barbed wire, and artillery repeatedly inflicted severe casualties on the attackers and counterattacking defenders. As a result, no significant advances were made. Among the most costly of these offensives were the Battle of Verdun with a combined 700,000 casualties (estimated), the Battle of the Somme with more than a million casualties (estimated), and the Battle of Passchendaele with roughly 600,000 casualties (estimated).
In an effort to break the deadlock, this Western Front saw the introduction of new military technology, including poison gas, aircraft and tanks. But it was only after the adoption of improved tactics that some degree of mobility was restored. The German Spring Offensive of 1918 was made possible by the Treaty of Brest-Litovsk that marked the end of the conflict on the Eastern Front. Using the recently introduced infiltration tactics, the German armies advanced nearly 60 miles (97 kilometres) to the west, which marked the deepest advance by either side since 1914 and very nearly succeeded in forcing a breakthrough.
In spite of the generally stagnant nature of this front, this theatre would prove decisive. The inexorable advance of the Allied armies during the second half of 1918 persuaded the German commanders that defeat was inevitable, and the government was forced to sue for conditions of an armistice. The terms of peace were agreed upon with the signing of the Treaty of Versailles in 1919.