re: US Military Domestic Response Times?

My apologies, Mr Grammar Nazi But I still contend it would be literally impossible to invade the US undetected with a large conventional force.

A more plausible scenario would be something in the Arctic. During the Cold War, there was a National Guard unit made up entirely of Eskimos, called the Eskimo Scouts who patrolled the far north. They would periodically find signs that the Russians had been there, and presumably, the Russians wanted them to know they had been there. I could see Putin starting trouble up there. Pinprick attacks by SOF forces trained in arctic warfare would be very difficult to prevent entirely.