Despite the belief that the war would be over quickly, why did a stalemate occur along the Western Front?

The correct answer to this open question is the following.

Despite the belief that the war would be over quickly, a stalemate occurred along the Western Front due to the fact that the German troops could not defeat France quickly and soldiers dug trenches. Germany assumed a defensive position during the stalemate of 1917.

The Allies, under the command of French leader Robert Nivelle, decide to move and attack the Germans, but they decide to withdraw to the Hinderburg Line. So plans changed, and the decision was to attack the Germans in the Battle of Arras of April 4, 1917, with the direct support of the British troops.

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