After President Hayes used federal troops to end the Great Railroad Strike of 1877, there was sufficient bipartisan support to pass what became the Posse Comitatus Act.

What is the history of Posse Comitatus?

The posse comitatus (from the Latin for "power of the county"), in common law, is a group of people mobilized by the conservator of peace – typically a sheriff – to suppress lawlessness or defend the county. The posse comitatus originated in ninth-century England simultaneous with the creation of the office of sheriff.

Is there an exception to Posse Comitatus?

Under the Insurrection Act, Congress provided an exception to the Posse Comitatus Act that allows the President to order military law enforcement in cases of rebellion that makes ordinary judicial enforcement of federal law impracticable.