Shay's rebellion in Mass, prior to the Constitution was put down by the State Militia, but the Whiskey rebellion slightly later was put down by Federal troops on orders of the President, George Washington, Federal troops, (US Marines, under command of Army Capt. R. E. Lee) put down the attempted rebellion of John Brown at Harper's Ferry. More recently the US Army was used in the stand off with American Indian Movement activists at Wounded Knee in 1973.
The best defensive strategy open to citizens is that used by the Reds against the Czar, educate the troops, make them aware that their duty is to PROTECT the citizens and constitution, not suppress them. Then when they receive unjust orders they can refuse to obey them or they can be killed or locked up, either by their comrades or the citizens.
All political power comes from the barrel of a gun, if the Govt. needs the guns to stay in power NOT firing is a powerful political force as well.
But the size of the projected force makes such worries foolish, we are talking about a force less than a 10th the size of NYPD. It may be enough to provide security in a relief effort in ONE location, but would not even be noticed if it tried to suppress the population, Even with LETHAL weapons the attempt would be a joke. Deploy that force in NY and they would be facing odds of 6667 to 1.
as to the legal tools for the Govt. to institute Martial law, they have been in place for over 200 years, modified under carter, and again under Bush, so it's no big deal.