The Bren Gun Literally Waged War During World War I and II While never completely replacing the Vickers, the Bren did serve as the primary support arm for British and Commonwealth troops through World War II and Korea. Here's What You Need To Remember: The Bren gun wasn't the best submachine gun of the war - but it was cheap to make, easy to use, and reliable to a fault. While all the combatant nations engaged in World War I fielded machine guns during the conflict, the British Army’s Vickers was arguably the best medium machine gun of the war, while their Lewis gun—an American design but perfected by the English—was the most effective light machine gun.