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.
Military strategy
There was still room for triumphs for the strategy of manoeuvre such as Sherman's March to the Sea in 1864, but these depended upon an enemy's unwillingness to entrench. Towards the end of the war, especially in defense of static targets as in the battles of Cold Harbor and Vicksburg, trench networks foreshadowed World War I. Many of the lessons of the American Civil War were forgotten, when in wars like the Austro-Prussian War or the Franco-Prussian War, manoeuvre won the day.