What does an army sacrifice when it commits to the protection provided by a trench

The answer is mobility. Trench warfare was popular at the end of the 19th and beginning of the 20th century, Trenches, essentially large ditches men dug, and then took up defensive positions in, were very good at giving soldiers a place to hide from incoming automatic weapons fire and mortar strikes. This however gave birth to "trench warfare", a style of combat relying on melee combat, short range weapons like shotguns, and worst of all, penetrating attacks the trenches couldn't protect from, such as shrapnel heavy grenades, flamethrowers, or even chemical weapon attacks. Trench warfare was also terrible as the living conditions in the trenches often involved standing water, disease, and malnutrition.

The french army was poorly prepared