The Insurrection Act overrides the Posse Comitatus Act. One of the Acts conditions is that 'unlawful obstruction' makes it 'impractical' to enforce federal law. This is how they used the Act to enforce desegregation. Trump can easily say that things like sanctuary state policy forced his hand. The states can sue, but the Supreme Court and a lot of federal judges will likely rule in his favor. It's gonna be ugly.
Ok, did some reading on The Insurrection Act and you are correct, it can over-ride the Posse Comitatus Act. A circumstance that would allow it is if the state or city decides it is going to countermand the federal order to allow ICE to come in and remove the illegal immigrants. So Sanctuary cities that decide their declared status allows them to ignore Federal Law and use their own governments to hinder the execution of the law, opens the door to him using The Insurrection Act to send the military in to aid in the execution of the law that is being obstructed by the local governments.
The law, which lets the president deploy the military domestically and use it for civilian law enforcement, is dangerously vague and in urgent need of reform.
www.brennancenter.org
"Section 253 has two parts.
The first allows the president to use the military in a state to suppress “any insurrection, domestic violence, unlawful combination, or conspiracy” that “so hinders the execution of the laws” that any portion of the state’s inhabitants are deprived of a constitutional right and state authorities are unable or unwilling to protect that right. Presidents Dwight D. Eisenhower and John F. Kennedy relied on this provision to deploy troops to desegregate schools in the South after the Supreme Court’s landmark decision in
Brown v. Board of Education."
So yes, if the states or cities decide they are going to fight the Federal government to protect the illegal aliens from ICE, he has a pathway to use that Act on those particular areas. It does not give him cart blanche authority to use the military in all areas of the US.
Also on the same website:
"However, while this precedent might prevent judges from second-guessing whether the president is allowed to invoke the Insurrection Act in response to a given situation,
the Supreme Court clarified in Sterling v. Constantin (1932) that courts may still review the lawfulness of the military’s actions once deployed. In other words, federal troops are not free to violate other laws or trample on constitutional rights just because the president has invoked the Insurrection Act."
So the rights of citizens can not be removed just because the President invoked the Act. Now if the courts will protect those rights can be argued since the Supreme Court is Right leaning though. Still, a pathway to head this off at the pass is to actually follow Federal Law, and not try to use your local government resources to obstruct the Federal Government from removing the Illegals.
Now, on this issue of removing Illegal Aliens, I have to admit I am biased on the issue. I see no problem with removing them from the country and forcing them to come back the right way. I am especially in favor of removing the major criminal element that has slipped into this country illegally, and also feel sanctuary cities should not exist for those here illegally. Stating this all to make my position clear on this issue.