How did new technology make defenses such as trenches necessary?

Answer 1

The machine gun had a huge impact. It could sweep all the area between two armies mowing down everything. Previously the rate of fire depended on the number of soldiers on each side and how fast they could fire individual shots - so the rate of fire was much lower. Trenches provided protection of an army from the small arms fire of the other side. Advances in artillery also made trenches advantageous. When an artillery shell fell, it would spread its damage great distances in all directions. Soldiers in trenches were largely protected from the explosions of the artillery shells which had to fall almost exactly on the trench and could only do damage laterally. Trenches were never straight but were dug in a zigzagging or stepped pattern. Later fighting trenches broke the line into firebays connected by traverses. This meant that a soldier could never see more than 10 yards (9 m) or so along the trench. Consequently, if a bomb or shell landed in the trench, the blast could not travel far.