There's more highly trained retired professionals.
No an AR isn't going to do much against an F16 at 20,000 feet. But all planes must land. Tanks are not the unstoppable metal beasts they are made out to be. Don't get me wrong, an AR isn't going to do anything against a MBT, however, the biggest weakness of a tank is their tracks. Take one out and it becomes combat ineffective. The crews are human and there isn't a bathroom in the tank. If you think an army only armed with small arms couldn't hold off, fight or defeat an army, modern history is replete with examples.