Posse Comitatus Act (18 U.S.C. § 1385)
Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air 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.
We generally assume that the Posse Comitatus Act means that the military simply can't be involved in any domestic law enforcement activities.
...but that is nothing compared to using the military, (especially a combat not MP) brigade as the police. Under the Posse Comitatus Act they’re not supposed to be able to.
However, there are quite a number of Acts of Congress that provide express exceptions to the Posse Comitatus Act.
LA riots in 1992: military intervention was under the terms of the Insurrection Act of 1807 (10 USC § 331
, 10 USC § 332
, 10 USC § 333
, and 10 USC § 334
Drug interdiction: various authority granted to the Secretary of Defense (32 USC 112
Military equipment: can be provided to assist law enforcement (10 USC 371-382