First, trench warfare was developed because the military noticed that they lose a lot of people because they fought in big numbers which led to more deaths when shot with rifles or other artillery. Mainly, because of the failure of the Schiefflen plan, trench warfare came into the picture. Since they were lacking in number and they have to spread in order to defend, they decided to devise a plan of infiltration and give them a surprise attack - but this failed. So, they built trenches to protect the military from being killed en masse.
