By invoking the Insurrection Act, Donald Trump can deploy the U.S military to quell violence at protests.
Under the U.S. Constitution, governors generally have the authority to maintain law and order within their states. This right comes under the law called the Posse Comitatus Act, which generally bars the federal military from participating in domestic law enforcement. But under the Insurrection Act, the president can authorize the federal military to enter state borders to quell domestic protests.
The Insurrection Act has been invoked a few times before in the United States. The Act was last used in 1992, when the acquittal of four Los Angeles police officers in the beating of black motorist Rodney King led to deadly riots, though in that instance, the state’s governor had requested it.