The original provision was enacted as Section 15 of chapter 263, of the Acts of the 2nd session of the 45th Congress.

Sec. 15. From and after the passage of this act it shall not be lawful to employ any part of the Army of the United States, as a posse comitatus, or otherwise, for the purpose of executing the laws, except in such cases and under such circumstances as such employment of said force may be expressly authorized by the Constitution or by act of Congress; and no money appropriated by this act shall be used to pay any of the expenses incurred in the employment of any troops in violation of this section and any person willfully violating the provisions of this section shall be deemed guilty of a misdemeanor and on conviction thereof shall be punished by fine not exceeding ten thousand dollars or imprisonment not exceeding two years or by both such fine and imprisonment.[15]

The text of the relevant legislation is as follows:

18 U.S.C. § 1385. Use of Army, Navy, Marine Corps, Air Force, and Space Force as posse comitatus: Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army, the Navy, the Marine Corps, the Air Force, or the Space Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.

Also notable is the following provision within Title 10 of the United States Code (which concerns generally the organization and regulation of the armed forces and Department of Defense):

10 U.S.C. § 275. Restriction on direct participation by military personnel The Secretary of Defense shall prescribe such regulations as may be necessary to ensure that any activity (including the provision of any equipment or facility or the assignment or detail of any personnel) under this chapter does not include or permit direct participation by a member of the Army, Navy, Air Force, or Marine Corps in a search, seizure, arrest, or other similar activity unless participation in such activity by such member is otherwise authorized by law.

Homeland Security Secretary Kristi Noem has asked Defense Secretary Pete Hegseth to instruct soldiers deployed to Los Angeles to help arrest rioters, even though the military is generally barred from domestic law enforcement.

In a leaked letter, Noem asked Hegseth on Sunday to have the Pentagon give “direction to [Department of Defense] forces to either detain, just as they would at any federal facility guarded by military, lawbreakers under Title 18 until they can be arrested and processed by federal law enforcement, or arrest them,” the San Francisco Chronicle reported.

Title 18 is the main code dealing with federal crimes and criminal procedure.

[...]

The main exception to the rule is the Insurrection Act, a compilation of statutes that allows the U.S. president to use the military to quash an armed rebellion or other insurrection, according to Lawfare.

[...]

The Insurrection Act was last used in 1992 to suppress the riots that broke out in Los Angeles following the acquittal of three white police officers who had severely beaten Rodney King, an unarmed Black man.

Those circumstances were far different, though. Dozens of people had been killed in the riots, and both the mayor of Los Angeles and the governor of California had asked then-President George H.W. Bush to mobilize the National Guard.