Answer: Trench Warfare
Step-by-step explanation:
Most of the battles that went on in the Western Front were in trenches. Trenches were deep, muddy pits with rooms dug into the ground that took a pretty long while to dig up and advancing from them into an enemy trench was a death sentence. Commanders would through several waves of soldiers into an enemy trench that would often get repelled until around the 5th try which would gain the soldiers either part of the front trench, or a good 200m, which is obviously not a lot. Now imagine this continued all around France and Belgium and multiply it by a couple hundred. Not very fast progress when you do that, huh?