All US aircraft and airfields are subject to militarization on a moments notice. It's an old Cold War law that's still on books. In NATO planning, virtually the entire private airliner fleet was going to be militarized to form an airbridge if the Russkies invaded.(It was assumed that nuclear attack subs would be way worse on ocean shipping than German diesel-electric were in WWII.)
Some airports are designated emergency military landing zones. Used to be this little grass stripped airport outside of Bangs, Tx that had to sweat to land an ultra-light but still had an official notice on the wall that it was expected to handle military aircraft during a designated emergency. I finally figured out they meant helicopters from the nearby National Guard training ground.