Background information: An explanation and overview of trench warfare in WWI.
Trench warfare refers the the tactic of of digging trenches along a battle line which opposes the trenches dug by the enemy. On signal, the soldiers of one side would come up out of their trenches and charge toward the enemy's line and attempt to gain ground by overrunning or overwhelming the enemy. The point was to gain ground and push the enemy back. Both sides would then dig new trenches. The area of ground between the opposing lines was known as "no man's land."
The idea of the trenches was to protect soldiers from small arms fire and machine gun fire. Chemical weapons were used. Chlorine gas was a favorite. The trenches were protected by barbed wire. It was miserable in the trenches when it rained. It rained a lot.
Trench warfare resulted in long stalemates and infrequent real gains. Sometimes when a trench was breeched by a charge, the enemy would counter attack the next day and drive the opposition back into their original trenches.
The usual procedure for an attack would be for the attacking side to open fire with artillery early in the morning, with the object being to soften up and shell-shock the enemy soldiers in their trenches. Artillery also cut the enemy's barbed wire, or so it was hoped.
Then the chlorine gas was released over the "no man's land" and over the enemy trenches.
Then the attackers, wearing gas masks which didn't allow for good vision or adequate breathing for a running man, would (with bravery I cannot begin to imagine) come up out of their trenches and charge through the chlorine gas over no man's land into the face of the enemy's machine guns. Failure was the result most often encountered. Sheer fatigue would set in. Men would throw off their gas masks in frustration and be killed or disabled by their own gas. As often as not, the wind would change directions and blow the gas back at the attackers anyway. This happened during the Battle of Loos.
How effective was this trench warfare? How dangerous was it? During the the fighting for Loos, a series of British charges began on September 25, 1915, and ended 3 days later. The British gained the enemy trenches and were very close to a breakthrough. They would have prevailed had replacements for their dead been available. That battle ended on September 28, with the British falling back to their original positions. They didn't have enough artillery ammunition. They had no reinforcements to replace the men devastated by machine gun fire on their flank. Due to lack of ammunition, their artillery failed to cut the enemy wire and failed to silence the machine guns. 140 tons of chlorine gas had been released and not all of it, by far, was being breathed by the enemy. The Scots broke through by sheer determination and force of numbers, but were being decimated.
During that single 3-day charge and retreat, the British took 20,000 casualties.
The unspeakable losses of trench warfare begat modern armor. Tanks.