The stalemate on the Western Front lasted for so long because it was much easier to defend the trenches than to attack them for the following reasons:
1. The weapons in the First World War were much better for defending than for attacking. As attacking soldiers charged across ‘No Man’s Land’ they were simply mowed down by machine gun fire or blown up by land mines. The defenders in the trenches were better protected.
Example: In the Somme, General Haig underestimated the machine gun and, as a result, 57,000 allied soldiers died on the first day.
2. ‘No Man’s Land’ was often deep mud and covered with barbed wire. This made it very difficult for men and horses to charge quickly.
For example: In the Somme, the tanks got stuck in the mud which added to the difficulty in trying to launch a successful attack.