A guy with a rifle is not a threat to any modern Army.
I agree that 1 guy with a gun isn't going to win a war. Hell, even a hundred. 1 guy can however cause a lot of chaos and confusion, and even shut down entire states for days.
Remember that it was just a few guys with rifles that had law enforcement and the general public shitting their pants every time that a loud noise was heard. Dorner had one of the largest police forces on the planet walking on egg shells. The DC sniper had most of new england scared shitless.
Natural laws have no pity.