That's the key. The NG would only have done any good if their rifles were loaded, and they had a commander willing to take the heat for killing a large pile of rioters.
Not going to happen.
Especially not for the benefit of the Republican mayor of Ferguson.
This is not difficult.
Simple deployment in riot gear without firearms would have sufficed. The Ferguson police could have done the firearms duty.
There weren’t a lot of rioters, just a few hundred it seems, of which a minority were semi-serious trouble. The NG could have easily outnumbered them.
Step 1 - Curfew
Step 2 - Roadblocks
Much of the problem was that the looters/vandals were rushing from location to location in their cars.
Step 3 - Guards at business premises. Freeing up the PD to arrest the violent.
Step 4 - Detainee cage operation/guards, again freeing up the PD.