A. To protect soldiers from machine gun fire is your best answer
While the Gatling gun had been around since the American civil war, machine guns had involved into more lethal weapons, as they were able to fire thousands of rounds at once, became lighter to carry, and were water-cooled. Trenches essentially was built to protect the soldiers from being massed-murdered by the machine gun. However, this isn't the only reason trenches were built, but one of them.
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