The idea of No Man's Land belongs to a time when armies had pitched battles. The area between the opposing lines was a place where neither side went. It was No Man's Land.

Part of the reason for this was that the armies kept the ground ahead of them clear, so they could see what was going on. They created a defence zone which they controlled in Medieval times by arrows, and in World War 1 (the time of the last pitched battles) by machine guns. By the time of World War 1, the invention of machine guns made No Man's Land a killing ground. When armies advanced over this area as part of an attack, they knew they had to shell the enemy machine gun posts out of existence before they could advance, or their losses would be very heavy. The video below shows what it was like for American troops on the front, with No Man's Land ahead of them.