During World War I, the second phase in battles that crossed enemy lines and approached trenches was heavy bombardment. sniper fire. aerial bombing. infantry attack.

The answer is infantry attack.

The infantry went over the parapet to attack the enemy trench line often with appalling losses as seen in the battle of the Somme in which the British suffered nearly 60,000 casualties or the battle of Verdun where the French suffered 380,000 casualties.

Toward the end of the war, allied forces invented and perfected tanks, turning the tide. Tanks had tremendous effects on the morale of German troops who had no answer to these terrifying machines. Allied troops easily broke through the German trench system and could not be dislodged through infantry counter attacks and harried the Germans back, using infantry supported by the tanks.