The fact that a large number of Americans would consider the "interveners" an invading army, and would take appropriate action.
The Dims like to ignore it, but they are keenly aware of the Second Amendment and what it means. It's why they want so dearly to gut it.
In any case, if this DID happen, the Dims would lose at least half the states, forever.
The dims keep poking the conservatives in the ribs, causing a distinct twitch that could result in CWII. They will try anything to win a race. At some point the gloves will come off.