You're assuming that he intended to invade the camp. But all he could do was to wait a safe distant outside and just stab an unsuspected soldier who was off duty. It's not like every person serving in the army is a special forces officer who's in alarm 24/7.
It's also important to note that, unlike the movies, most bases don't actually have a lot of people walking around armed. Combat gear is stowed and firearms are in the armory under lock and key. Most jobs don't actually require carrying a weapon while on duty. Even those that do need some sort of weapon don't require such at all times.
The only time I regularly carried a weapon was when I was a transportation officer/chaser. I carried a .45 and a 12ga and only when I was performing duties that required such, namely while actively participating in the transfer of detainees.
Keep in mind that my first four years was in the infantry and included combat. I probably carried a weapon a smaller percentage of the time with that MOS than I did while working at the brig.
