Barbed wire entanglement were often 40 yards (36m) from the front trench considering the types of weapon used on the western front, what would be their main defensive function

The main function was to create a line of defense for the trenches.

Explanation:

Barbed wire was essential during the "trench war" which is the first world war.

This is because the army dug trenches in the ground so that soldiers could attack the enemy army while they were inside those trenches. Between the trenches of each army, there was a free space called "no man's land" that was used for soldiers to leave their trenches and enter the enemy trenches. To prevent this, barbed wire was used to protect the trenches and provide an obstacle for enemy soldiers.

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