What was the area in between opposing trenches called during world war 1

 "No man's land."

Context/explanation:

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 All Quiet on the Western Front 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.

b: to eliminate competition from other farmers