Industrialization and War.
World War I was the first war that used modern weapons and warfare. It is a result of industrialization. Because of industrialization, countries invented new ways to engage in war. In previous wars, most of the fighting was through the use of single shot rifles, short range cannons, and hand to hand combat. However, in World War I, new inventions were used that will alter warfare forever. These inventions include:
Machine Gun
The hand-held grenade
Tank
Trench Mortars
Poison Gas (Mustard Gas)
Because of these new weapons, Armies could not fight in the traditional manner. As a result, the Armies of World War I "dug-in" and built massive trenches. This is the start of "trench warfare."
Trench warfare is not new. In fact, it was used extensively during the American Civil War. However, the trenches of World War I brought the idea to another level. These trenches housed millions of soldiers and were very complex. The German trenches even had electricity and running water. Most of the trenches' conditions were horrendous and massively dangerous. This type of engagement caused World War I to essentially become a "stalemate war" where there was very little movement.