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.
Trench warfare
A recent example of trench warfare is the War in Donbass. To consolidate and defend their territory, government and rebel forces have dug trenches and have initiated warfare reminiscent of World War I, with troops staying in and fighting from trenches for months at a time using cement mixers and excavators at their lines for digging network tunnels and deep bunkers for protection. Since the Minsk peace agreements the front lines have not moved significantly and the two sides mostly fire mortars and sniper shots at each other. As a result both sides have dug elaborate networks of trenches and deep bunkers for protection.