Answer:
Mainly technology.
Step-by-step explanation:
WWI was the first truly industrialized war, with heavy industry supporting both sides, internal combustion engines playing a major role in the air and on the ground in tanks, technological developments were made during the war, and machine guns bringing about a stalemate on the Western Front.
Explosive devices were more powerful, and artillery became the greatest killer on the Western Front. Innovations were deadly: Poison gases were utilized on a large scale, flamethrowers were invented, airplanes bombed and strafed and shot each other down, and submarines (U-boats) played a huge role.