Why did the Western Front become a maze of trenches?

The Western Front became trenches because defending had suddenly become far easier than attacking. By 1914 rapid-firing artillery, machine guns and high-explosive shells gave defenders an overwhelming advantage over soldiers walking or riding towards them. When armies could no longer win by charging, they did the sensible thing: they dug in.

Both sides tried to get around each other's flank, and each attempt just stretched the line further, until it formed one continuous line of trenches from Switzerland to the North Sea. By late October 1914 the front was stalemated. With no open flank left to go around, the only way forward was a frontal assault, and those were costly for attackers and defenders alike.

A trench was not one ditch. British doctrine called for three parallel lines, front, support and reserve, joined by communication trenches. In front lay no man's land, typically 90 to 275 metres wide, and belts of barbed wire more than 15 metres deep. Wire combined with machine-gun fire caused the majority of failed assaults.

Life there was mostly waiting. A unit usually spent between a single day and two weeks in a front-line trench before being relieved. By day, snipers and observers made moving around perilous, so the trenches were quiet; at night men moved supplies and mended the wire. The biggest killer was artillery, which caused around 75 percent of casualties. Trench fever, spread by lice, infected over one million Allied troops. The deadlock lasted until March 1918.