President Trump’s mobilization of the National Guard in several U.S. cities in recent months is a departure from its historic role.

The National Guard is a state-based military force whose troops wear uniforms and rank, operate military vehicles and weapons, and have often been called upon to serve alongside regular active-duty units in war. But when deployed domestically, the Guard has traditionally been asked to provide humanitarian assistance, not to act as a partisan strike force at the whim of the president.

Mr. Trump has activated the Guard several times this year in Democratic-led cities, in what he has called a crackdown on crime and as part of his escalating immigration enforcement tactics.

Here is an overview of what the National Guard typically does and how Mr. Trump is currently deploying the force in the United States.

What is the National Guard?

Tracing its history to the Massachusetts militia in 1636, the National Guard is a component of the U.S. military that has state and federal missions and traditionally operates under the jurisdiction of governors under Title 32 of the U.S. Code.

By comparison, the active-duty military operates under Title 10 of the U.S. Code, and the 1878 Posse Comitatus Act makes it a crime to use the regular military as a domestic police force in most circumstances.