In addition to the FBI, ATF, DEA, and other federal departments with a police "arm" don't forget Posse Comitatus isn't a complete ban.
The Navy has what appears to be a nice little write-up here about the history of the act, and the exemptions to the act currently in play.
Here is a brief history/theory from the cases born from Wounded Knee in 73, and changes since then.
An excerpt from the first link (A US government site, and thus public use I believe) says:
Second, over the years it has passed a fairly extensive array of particularized statutes, like those permitting the President to call out the armed forces in times of insurrection and domestic violence, 10 U.S.C. §§ 331-335.
This is because of what I
think is the
Insurrection Act of 1807 which provides:
Whenever the President considers that unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States, make it impracticable to enforce the laws of the United States in any State by the ordinary course of judicial proceedings, he may call into Federal service such of the militia of any State, and use such of the armed forces, as he considers necessary to enforce those laws or to suppress the rebellion.
which I assume is at least part of what allowed Eisenhower to federalize the Arkansas National Guard and use them to enforce the integration of Central High School.
Cooper V Aaron also suggests violating their oath to support and defend the Constitution is warring against the Constitution. I don't think it would be difficult to convince people warring against the Constitution is insurrection.
Edited because I hit post instead of preview:
Finally we have
10 U.S. Code § 311 which appears to be part of the Dick Act, AKA The Militia Act of 1903.
(a) The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard.
(b) The classes of the militia are—
(1) the organized militia, which consists of the National Guard and the Naval Militia; and
(2) the unorganized militia, which consists of the members of the militia who are not members of the National Guard or the Naval Militia.
So if Missouri, or one of the other States that tried to nullify really wants to get in a Big Johnson contest, their State National Guard can be federalized, and shipped off to USAG Mannheim, if it isn't willing to support the federal police agencies in state. Afterwards another State's Federalized Guard, or the active-duty military itself can be sent into the nullifying State. They may or may not be able to participate in arrests and other seizures, but I would suspect they can. Finally, the figurative "nuclear option" would be to draft/call up every able-bodied male citizen or prospective citizen 17-45 in the unorganized militia and ship them off.
Of course, like our real nuclear option during the cold war, this is most likely mutually assured destruction. ABC/FOX/CBS/NBC running film of the Army rounding up men 17-45, and shipping them off to Alaska for their "militia training and indoctrination" isn't going to play well anywhere, but if the i's are dotted, and t's are crossed, a Lame Duck President isn't going to care very much about his re-election chances.
There is no winner in this sort of stand-off. Nor does Missouri even WANT to win. I'm sure they like the sound of nullification, but they also like the sound of tacking on federal charges or transfers to federal custody in the right situations.
