From seizing control of the internet to declaring martial law, President Trump may legally do all kinds of extraordinary things.
The moment the president declares a “national emergency”—a decision that is entirely within his discretion—more than 100 special provisions become available to him. While many of these tee up reasonable responses to genuine emergencies, some appear dangerously suited to a leader bent on amassing or retaining power. For instance, the president can, with the flick of his pen, activate laws allowing him to shut down many kinds of electronic communications inside the United States or freeze Americans’ bank accounts. Other powers are available even without a declaration of emergency, including laws that allow the president to deploy troops inside the country to subdue domestic unrest.
The Atlantic At the heart of the debate is the Posse Comitatus Act, a much misunderstood 1878 law that allows for far greater use of the military in national emergencies than many believe.
Court opinions have clarified what the act does and does not prohibit. First, it applies only to the active duty military and not to the National Guard, unless the Guard is federalized by the president. The Guard normally operates under state, not federal, law, and is answerable to the respective governor rather than the president. Unless there is some restriction in state law restricting use of guard forces in a law enforcement function, they can properly be used in that capacity at the call of the governor.
Mere surveillance by military personnel has also been held not to violate the act.
Lastly, by its own terms, the Posse Comitatus Act does not restrict using active duty armed forces in law enforcement activities where "authorized by the Constitution or Act of Congress."
Duke.edu Nevertheless, there is substantial public debate and uncertainty around whether Sections 1021 and 1022 of the NDAA could be read even to repeal the Posse Comitatus Act and authorize indefinite military detention without charge or trial within the United States.
ACLU Navy and Marines. The Posse Comitatus Act proscribes use of the Army or the Air Force to execute the law. It says nothing about the Navy, the Marine Corps, the Coast Guard, or the National Guard. The courts have generally held that the Posse Comitatus Act by itself does not apply to the Navy or the Marine Corps. They maintain, however, that those forces are covered by similarly confining administrative and legislative supplements, which appear in the Department of Defense (DoD) Directive.
Navy.mil For those interested in learning more:
President Trump Declares National Emergency Manpower Guidance: Activation of USMCR in Support of Civil Authorities