Ok, so the "Great war" was called the war to end all wars because:
  The answer is really quite simple. World War 1 was a terrible shock to everyone involved. Only a few largely ignored people really had the foresight to predict how the war would play out. World War 1 was the first war in which nation states could take multiple hits and still keep fighting.
Before World War 1, battles generally lasted probably 1–3 days at the most. Some battles in World War 1 would last for months, some of them never ended and sort of molded into multiple battles.
Before World War 1, the worst battles in the memory of the European powers were ones where 5,000 to maybe 9/10,000 men were killed. During the opening exchanges of the war, it wasn’t unusual to lose that many men in a single day. The worst battles of Verdun and the Somme during the height of Trench Warfare saw hundreds of thousands of men die. Still people could not believe it was happening, and more so they still didn’t believe it was sustainable, but it was.
Before World War 1 most wars were not static. Armies would meet on the field of battle, have a few blows and either retreat or advance. The fighting men would not have to endure the horror of sitting in the same trench for months and years where generations of soldiers had died, and the barren apocalypse of war was all around them with fields of corpses, barbed wire, crater holes. Infestations of rats lice, bogs of mud which would suck a man into them and pools of water contaminated with the residue of mustard gas. To be wet, cold and mentally broken as they sat there enduring hours upon hours of earth shattering artillery bombardment at a time.
