The thing I don't think people really grasp here is that this that they very well likely have big money behind them. This area I grew up in here has like 23 billionaires here, and they are often pretty right wing to the extreme, they have all sorts of crazy things on their land here. They do not need to buy old stock. They can buy whatever they want. They aren't fighting the US military, they ARE the US military. They are recruiting from within the military. The FBI has been telling us this for decades. These guys are ALSO active US military.I am aware of that. I am also keenly aware that there's a massive difference between a $35M AH-64 and jury-rigging a pair of machine guns to a Bell Jet Ranger. There's also a massive difference between a vintage M4 Sherman or M48 Patton (or even a vintage first generation M1 Abrams if you can somehow get a hold of one) and a military grade M1A2 Abrams with modern upgrade kits.
What needs to be understood in these discussion is that owning a military grade vehicle is legal, but you won't be able to get a hold of one that's in contemporary use by the US military. And even if you could get a hold of a state of the art AH-64 attack helicopter a civilian would never be able to attain all the support structures needed to effectively use one. There's, very literally, hundreds of people employed just to ensure that one attack helicopter or fighter jet can function as the disgustingly effective weapon it is. It requires people with highly specialized skill sets (aircraft technicians, radar operators, intelligence analysts, pilots, munitions specialists etc.) using highly specialized tools to ensure that an AH-64 isn't just a cool aircraft hovering around but a weapon so potent that it rivals, or even exceeds, an armored battalion from WW2 in destructive capability.
The Army of Iraq, as pitiful as it was, was much, much better equipped then any militant group in the US can realistically hope to be and it got absolutely massacred two times over by the US military. If the US military wants to have you dead and you are stupid enough to engage in open combat you are very, very dead. The best a militant group like the one in your OP can hope to do is to engage in domestic terrorism in order to destabilize the US over a long period of time. Because if sovereign states using large amounts of military hardware they are proficient with can't even put up a fight against the US military, you can bet that some police officers, veterans and preppers with too much gusto and not enough brains don't stand a chance.

Signs of white supremacy, extremism up again in poll of active-duty troops
More than half of minority service members have seen links to the hate groups among fellow troops.


FBI Reports on Extremists in Military
White supremacist leaders are making a concerted effort to recruit active-duty soldiers and recent combat veterans of the wars in Iraq and Afghanistan, according to a new FBI report.
www.splcenter.org

White supremacists and militias have infiltrated police across US, report says
A former FBI agent has documented links between serving officers and racist militant activities in more than a dozen states

SPLC Testifies Before Congress on Alarming Incidents of White Supremacy in the Military
The SPLC’s Lecia Brooks testified today before the Subcommittee on Military Personnel (U.S. House Armed Serviced Committee) about white supremacy in the military and how to stop it. Below is Brooks’ written testimony. Thank you for the opportunity to submit testimony to the U.S. House Armed...
www.splcenter.org