The military response to protest and the law
By 
Good explainer from Charlie Savage [gift link]. The whole thing is worth reading but the section on rules of engagement is particularly important (and harrowing):
This is unclear.
Stephen I. Vladeck, a Georgetown University law professor, wrote in a Substack post analyzing the order that for now the federalized troops appeared to have limited authority. Once deployed, they will be able to protect ICE agents and federal buildings against attacks by protesters but not to carry out immigration raids or police the city’s streets in general.
But Mr. Trump’s order did not specify any standards for when troops would be able use force — like arresting people or shooting them — if his administration deemed a protest to threaten federal personnel, property or functions.
Notably, Mr. Hegseth has railed against military lawyers who promoted what he saw as unduly restrictive rules of engagement aimed at protecting civilians in war zones. He has fired the top judge advocate general lawyers who give advice on legal constraints. And his remarks on Saturday and Sunday about using troops in Los Angeles have not signaled restraint.
In a social media post, Mr. Hegseth called protests against ICE in Los Angeles “violent mob assaults” designed to prevent the removal of undocumented migrants who he said were engaged in an “invasion.”
Hina Shamsi, the director of the American Civil Liberties Union National Security Project, said on Sunday that “no matter who carries the gun or what uniform they wear, it’s important to remember that the Constitution — and in particular the First Amendment — applies and troops’ conduct is governed by strict constitutional limits.”
Trump’s pardoning not only of the 1/6 insurrectionists but war criminals is critical context here.
On the relevant legal authority:
Usually it is not, but sometimes it can be.
Under an 1878 law called the Posse Comitatus Act, it is normally illegal to use federal troops on domestic soil for policing purposes. But an 1807 law, the Insurrection Act, creates an exception to that ban for situations in which the president decides that “unlawful obstructions, combinations or assemblages, or rebellion against the authority of the United States,” make it “impracticable” to enforce federal law.
Mr. Trump’s order criticized the protests as violent and said they threatened to damage federal immigration detention facilities, adding that, “to the extent that protests or acts of violence directly inhibit the execution of the laws, they constitute a form of rebellion against the authority of the government of the United States.”
Mr. Trump invoked a statute, Section 12406 of Title 10 of the U.S. Code, that allows him to call National Guard members and units into federal service under certain circumstances, including during a rebellion against the authority of the federal government. But he did not invoke the Insurrection Act.
The call-up statute does not, on its face, confer any authority to use federal troops in the ways Mr. Trump authorized Mr. Hegseth to use them. But Mr. Trump also referred to “the authority vested in me as president by the Constitution,” which may mean his administration thinks he can claim inherent power as president to use troops on U.S. soil in those ways.
Notably, during the Vietnam War, William H. Rehnquist, the future Supreme Court justice, wrote memos for the Justice Department Office of Legal Counsel saying that presidents had inherent power to use troops to prevent antiwar protesters from obstructing federal functions or damaging federal property in Washington, D.C., and at the Pentagon.
When a Republican administration wants to do something lawless, Nixon’s most Nixonian nominee Bill Rehnquist tends to be lurking in the background somewhere.