Trench warfare in World War I was miserable and gruesome. The armies had dug into trenches across from one another, and any attempts to rush out and attack the other side usually meant getting mowed down by machine gun fire. You might want to check out <em>All Quiet on the Western Front</em> by Erich Maria Remarque (1929) for first-hand descriptions of the misery of the trench warfare.
The term "no man's land" meant that the area between the safety of the opposing trenches was no place to be. You were likely to get slaughtered if you ventured out there.
<em>A sidenote for sports fans: If you're a tennis player, the term "no man's land" is used also in tennis, a throwback to the terminology of World War I. A tennis player wants to be hitting ground strokes from the back of the court, or else be all the way up at net to hit volleys while at net. If a player gets caught in the middle of his side of the courr -- "no man's land" -- his opponent will hit the ball right at his feet and make it impossible for him to hit a return. </em>
