After the Civil War, the Insurrection Act was further amended to give the president authority to enforce the 14th Amendment and the conditions of Reconstruction in the South. That authority is now found in Section 253 of Title 10 of the U.S. Code, which gives the president the right to take military action within a state when “any part or class of its people is deprived of a right, privilege, immunity, or protection named in the Constitution and secured by law, and the constituted authorities of that State are unable, fail, or refuse to protect that right, privilege, or immunity, or to give that protection.”
That’s the same authority invoked during the civil rights era by Presidents Eisenhower and John F. Kennedy to deploy troops to the South to enforce desegregation in defiance of governors.
The Insurrection Act was last invoked in 1992 under President George H.W. Bush, after Peter Wilson, then-governor of California, requested help to quell widespread riots after four police officers charged in the beating of Rodney King were acquitted.
In 2005, after Hurricane Katrina devastated Louisiana and the Gulf Coast, President George W. Bush explored expanding the Insurrection Act to place command of the region's National Guard under federal control. Ultimately, Bush declined to invoke the act, although it was eventually amended in 2006 to broaden the scope under which the president may act under the law.