As already noted, Article IV endows the president with unilateral authority to put down insurrection and invasion. “Insurrection,” of course, is a violent uprising against the authority of the state. Therefore, it encompasses the concept of broad-scale domestic violence. The Insurrection Act codifies this reality by expressly empowering the president to suppress domestic violence and conspiracies to carry it out.
It is a fair concern that modern Insurrection Act expansions of federal power to react to natural disasters overrun federalist principles — although, as a practical matter, when states actually confront a natural disaster or pandemic, governors are quick to plead for federal assistance, rather than fret over what doing so portends for their power over their own affairs. When it comes to violent uprisings, though, federalism concerns are beside the point. Washington has an express constitutional obligation to protect the people of the states, and the republican form of government, from attack.
Federal power in this regard is also reflected by the president’s authority over the National Guard. The Guard is composed of state units, an arrangement with its roots in the colonial militias that were called into the service of the Continental Army during the Revolutionary War.