Answer:
The use of machine guns resulted in massive casualties
Step-by-step explanation:
On the western front, soldiers dug into trenches and would stay there until they were ordered to cross "no man's land". It was called this because of the machine guns, barbed wire, mines and the open field they had to get through to make it across.
Nobody wanted to cross no man's land because a single machine gunner could pin down and dispatch many soldiers in a short period of time. This resulted in long stalemates where neither side wanted to cross.