Is Trump stretching the law to deploy federal police power in cities?

The federal government has broad power to enforce the laws of the United States, but not to police the streets or maintain order in a city if protests lead to violence.
That has been how the separation of powers between states and the federal government has been understood. The Constitution leaves the “so-called police power” in the hands of state and local officials. It is one of the “powers not delegated the United States” and instead is “reserved to the states,” as the 10th Amendment says.

This principle has...