The Posse Comitatus Act passed on June 18, 1878, prevents the military from engaging in civil law enforcement, but the exception came with The Insurrection Act.
“Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That in all cases of insurrection, or obstruction to the laws, either of the United States, or of any individual state or territory, where it is lawful for the President of the United States to call forth the militia for the purpose of suppressing such insurrection, or of causing the laws to be duly executed, it shall be lawful for him to employ, for the same purposes, such part of the land or naval force of the United States, as shall be judged necessary, having first observed all the pre-requisites of the law in that respect,” it says.
Here is how the president could declare it. You may have noticed in the news recently that protests for and against President Trump have turned into fights bordering on riots.
A 2006 Congressional Research Service report the president “must first issue a proclamation ordering the insurgents to disperse within a limited time, 10 U.S.C. § 334.4. If the situation does not resolve itself, the President may issue an executive order to send in troops.”
