“The President of the United States has zero authority without violating the Constitution to call the National Guard to serve outside of their State borders.”
30,000 National Guard troops from several states served in New Orleans after Katrina. The military police presence lasted for years in the city.
There was that whole WW1 thing; WW2, Korea, Vietnam, Desert Storm, Iraq, and Afghanistan too.
Actually, the president did not call us up for Katrina. That was a interstate agreement between the States. We were federalized (title 32) while we were there. Full military pay beats $5 and found.