You can own tanks and jets as long as the weapons are removed. Some of those weapons are still legal though. I wouldn't doubt there is a number of legally owned tanks and jets that could be converted to military ready in just a few days here in the US.

Also just look at Iraq and Afghanistan, the fighters over there kill US soldiers all the time despite being very poorly equipped. The US general population is far better equipped and more numerous than the Taliban and other terrorist groups. If the US can't control small areas over there, it seems extremely likely they would lose all control here in the US.