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.
Horses in warfare
Early in the War, cavalry skirmishes were common, and horse-mounted troops widely used for reconnaissance. On the Western Front cavalry were an effective flanking force during the "Race to the Sea" in 1914, but were less useful once trench warfare was established. There a few examples of successful shock combat, and cavalry divisions also provided important mobile firepower. Cavalry played a greater role on the Eastern Front, where trench warfare was less common. On the Eastern Front, and also against the Ottomans, the "cavalry was literally indispensable." British Empire cavalry proved adaptable, since they were trained to fight both on foot and while mounted, while other European cavalry relied primarily on shock action.