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.
Experimental Mechanized Force
In the aftermath of the First World War, several theorists sought ways to avoid a repetition of trench warfare, despite the war of movement from August to December 1914 costing the French and the Germans The trench warfare that followed had been less costly in men but attrition warfare was indecisive; limited objective attacks under an umbrella of massed heavy artillery-fire could succeed but only at the cost of unlimited duration. Colonel J. F. C. Fuller, formerly the Chief of Staff of the Tank Corps, proposed an all-tank force, which would operate independently against enemy headquarters and lines of communication. Less radical theorists such as the historian and former British army officer Basil Liddell Hart, advocated mechanised forces of all arms, able to carry out operations of war other than the all-out offensive. Giffard LeQuesne Martel, a third influential reformer, proposed that tanks should be subordinated to infantry formations, while many cavalry officers maintained that the horse still had a part to play on a modern battlefield, in spite of all evidence to the contrary on the Western Front in the First World War.