Unfortunately with the invention of mustard gas, the trenches also became deadly traps. Mustard gas was heavier than air and would sink into the trenches and stay, poisoning all of the soldiers in that trench.

Trenches, pill boxes, etc., are defensive structures. The different fronts had networks of trenches for troops to take cover in (from artillary, etc.), and as places to mass together, prior to coming out of the trenches to advance (attack) on the enemy lines.

Trenches have been used in previous and subsequent wars – as long as the troops were in place long enough to build them, and there was a need.

The bases I was at in Vietnam in 1967/68 did not have trenches (lots of bunkers though), but I am sure Khe Sanh and the earlier Dien Bien Phu did use trenches because they were under intensive artillery and rocket attack for months, so the only way one would have been able to move around there would have been with trenches or tunnels.

Instead they simply marched their men straight into machine gun and artillery fire. The leadership didn’t know how to cope with having entire divisions wiped out in a single battle. Tanks came later and there was no effective air power support strategy like now. As a result they resorted to trench warfare.

Ironically it was the French who rediscovered maneuver warfare in taking out machine gun nests and their crews.