Since the British and French forces were alert to the German flamethrowers, when the German flamethrowers shot, the British and French forces focused their fire on them.
As the British and French allied forces were already alert to the German flamethrowers, when the German flamethrowers fired, the British and French allied forces focused their fire on them
Because the Anglo-French allied forces had been alert to the German flamethrower, when the German flamethrower fired, the Anglo-French allied forces concentrated their fire on it.