When Union Army General William Tecumseh Sherman famously said “War is hell,” he was referring to war in general, but he could have been describing trench warfare, a military tactic that’s been traced to the Civil War. Trenches—long, deep ditches dug as protective defenses—are most often associated with World War I, and the results of trench warfare in that conflict were hellish indeed.

Trench warfare in World War I was employed primarily on the Western Front, an area of northern France and Belgium that saw combat between German troops and Allied forces from France, Great Britain and, later, the United States.

Although trenches were hardly new to combat: Prior to the advent of firearms and artillery, they were used as defenses against attack, such as moats surrounding castles. But they became a fundamental part of the strategy with the influx of modern weapons of war.

Long, narrow trenches dug into the ground at the front, usually by the infantry soldiers who would occupy them for weeks at a time, were designed to protect World War I troops from machine-gun fire and artillery attacks from the air.

As the “Great War” also saw the wide use of chemical warfare and poison gas, the trenches were thought to offer some degree of protection against exposure. (While significant exposure to militarized chemicals such as mustard gas would result in almost certain death, many of the gases used in World War I were still relatively weak.)

Thus, trenches may have afforded some protection by allowing soldiers more time to take other defensive steps, such as putting on gas masks.

At least initially in World War I, forces mounted attacks from the trenches, with bayonets fixed to their rifles, by climbing over the top edge into what was known as “no man’s land,” the area between opposing forces, usually in a single, straight line and under a barrage of gunfire.

Not surprisingly, this approach was rarely effective and often led to mass casualties.

Later in the war, forces began mounting attacks from the trenches at night, usually with the support of covering artillery fire. The Germans soon became known for effectively mounting nighttime incursions behind enemy lines, by sending highly trained soldiers to attack the trenches of opposing forces at what they perceived as weak points.

If successful, these soldiers would breach enemy lines and circle around to attack their opponents from the rear, while their comrades would mount a traditional offensive at the front.

The brutality of trench warfare is perhaps best typified by the 1916 Battle of the Somme in France. British troops suffered 60,000 casualties on the first day of fighting alone.

With soldiers fighting in close proximity in the trenches, usually in unsanitary conditions, infectious diseases such as dysentery, cholera and typhoid fever were common and spread rapidly.

Constant exposure to wetness caused trench foot, a painful condition in which dead tissue spread across one or both feet, sometimes requiring amputation. Trench mouth, a type of gum infection, was also problematic and is thought to be associated with the stress of nonstop bombardment.

As they were often effectively trapped in the trenches for long periods of time, under nearly constant bombardment, many soldiers suffered from “shell shock,” the debilitating mental illness known today as post-traumatic stress disorder (PTSD).

It’s likely all of these factors, which stemmed from the widespread use of trench warfare, made World War I the deadliest conflict in global history to that point. It’s believed that as many as one in 10 of all fighting forces in the conflict were killed.

It was also the first conflict in world history to have more deaths caused by combat, rather than from disease spread during the fighting.

Trench warfare was also employed in World War II and in the Korean War to some degree, but it has not been used regularly during conflicts in the ensuing decades.