Armies have been storming trench lines for more than a hundred years, but for all the advances in military technology, it is no less harrowing now than it was when soldiers were crossing the muddy battlefields of World War I.

Assaults can be stealthy and surgical, employing surprise, or launched with overwhelming force, using drone strikes, or tanks and artillery. The goal is to breach a maze of protected firing positions and bunkers linked by sunken walkways and guarded by the enemy.

Ukrainian soldiers chose the louder option for an assault on a trench line in May. Some who participated described a fast, well-choreographed operation of a type that is likely to play a pivotal role in Ukraine’s long-anticipated counteroffensive, which U.S. officials suggested on Monday has begun, and which will entail breaking through belts of Russian land mines, tank barriers and trenches.

During the assault in May, Ukrainian mortar units bombarded the position. A tank rumbled out and opened fire. Then armored Humvees bounced forward over a field, firing machine guns, the men said. The assault group fired 3,000 bullets from two American-provided Browning machine guns, one commander named Kozak said, in an indication of the immense ammunition requirements for troops on the offensive.