Trench warfare
Trench warfare prevailed on the Western Front from late 1914 until the Germans launched their Spring Offensive on 21 March 1918. After the buildup of forces in 1915, the Western Front became a stalemated struggle between equals, to be decided by attrition. Frontal assaults, and their associated casualties, became inevitable because the continuous trench lines had no open flanks. Casualties of the defenders matched those of the attackers, as vast reserves were expended in costly counter-attacks or exposed to the attacker's massed artillery. There were periods in which rigid trench warfare broke down, such as during the Battle of the Somme, but the lines never moved very far. The war would be won by the side that was able to commit the last reserves to the Western Front.
Battle
Until the 19th century the majority of battles were of short duration, many lasting a part of a day. (The Battle of Preston (1648), the Battle of Nations (1813) and the Battle of Gettysburg (1863) were exceptional in lasting three days.) This was mainly due to the difficulty of supplying armies in the field, or conducting night operations. The means of prolonging a battle was typically by employment of siege warfare. Improvements in transportation and the sudden evolving of trench warfare, with its siege-like nature during World War I in the 20th century, lengthened the duration of battles to days and weeks. This created the requirement for unit rotation to prevent combat fatigue, with troops preferably not remaining in a combat area of operations for more than a month. Trench warfare had become largely obsolete in conflicts between advanced armies by the start of the Second World War.