Trench warfare
Trench warfare occurred when a revolution in firepower was not matched by similar advances in mobility, resulting in a grueling form of warfare in which the defender held the advantage. On the Western Front in 1914–1918, both sides constructed elaborate trench, underground, and dugout systems opposing each other along a front, protected from assault by barbed wire, mines, camouflaged trapping pits, and other obstacles. The area between opposing trench lines (known as "no man's land") was fully exposed to artillery fire from both sides. Attacks, even if successful, often sustained severe casualties.
World War I
After the First Battle of the Marne (5–12 September 1914), Allied and German forces unsuccessfully tried to outflank each other, a series of manoeuvres later known as the "Race to the Sea". By the end of 1914, the opposing forces were left confronting each other along an uninterrupted line of entrenched positions from Alsace to Belgium's North Sea coast. Since the Germans were able to choose where to stand, they normally had the advantage of the high ground; in addition, their trenches tended to be better built, since Anglo-French trenches were initially intended as "temporary," preparatory to breaking the German defences.