The War on Land

The Great War was the most vicious war of all time when it occurred.
Lots of new technology appeared, such as machine guns, quick and long-ranged artillery, toxic gasses and tanks.
As soldiers and officers were unfamiliar with this new technology, casualties were very high.
Modern weapons and new technology with old tactics led too lots of deaths.

Trench Warfare and the Western Front

When the German offensive came to a halt, both sides began to dig trenches.
Trenches would be very effective at defense and would make attacks very difficult.
The Western front's frontline became mostly stationary because of the powerful defenses.
The fronts did not change much during the war other than the occasional successful offense or tactical retreat.
These trenches protected soldiers from enemy fire and eventually living quarters and bunks would also be dug out.

Trench areas would be formed from several lines of trenches, with machine gunners towards the frontmost trench and heavy artillery at the back.
Many sniper nests would also be scattered throughout.

For example these carved-out trees that would house sniper positions within them.
In-between the trenches was No Man's Land.
No Man's Land was filled with craters and barbed wire and other obstacles, meaning it was difficult to traverse.
Soldiers on an assault would be mowed down by machine guns.
When one side went on an offensive they would mostly be killed by the defending side, which attacked the other side afterwards, faced casualties and rinse and repeat.
Poor trenches were often flooded, were covered in waste and infested with rats. Especially the poorly made British or French trenches.
A bacterial disease called trench foot would often debilitate or kill soldiers. If not treated it would usually cause death (the disease is more formally called gangrene, and can cause lethal sepsis).
Toxic gasses were used in the war to deadly effect.
When first used these were surprising and lethal.
Mustard gas and Chlorine gas were used in the war.
Before being given gas masks some British soldiers used cloths soaked in urine as rudimentary filters.
Additionally toxic gases were denser than air, and would gather in craters and ditches in the already precarious No Man's Land, making it worse than it already was

The Attack of the Dead Men or the Battle of Osowiec Fortress was a battle during the Great War between Russia and Germany.
German troops tried to kill out the Russian soldiers in the heavily fortified fortress using toxic gas, however the Russian troops (some coughing blood or others using soaked cloths as masks) went on a vigorous assault.
The Russians managed to repel the attack due to German forces panicking at the sight of the Russian men covered in blisters and coughing up parts of their lungs.
The information isn't important but it does give a perspective on the horrors of the Great War.