Complex sentence: Prior to the attack, the guns were moved approximately 1 mile away; however, the concrete fortifications were intact, and would still present a major threat to the landings if they were occupied by artillery forward observers.
Split into simpler sentences: Prior to the attack, the guns were moved approximately 1 mile away. However, the concrete fortifications were intact, and would still present a major threat to the landings if they were occupied by artillery forward observers.