My understanding of a US military action against North Korea (time frame mid-September to approx. 10th of October, 2017) is NOT an invasion or a nuclear attack; I believe the attack will be by US missiles, drones and warplanes, that the North Korean airfields, missile launch sites, missile factories and nuclear weapons facilities will be hit, and that any fire by North Korea canon emplacements into South Korea will be hit by US missiles, planes or drones.
This is my understanding a a "war" with North Korea; it would probably be no more than a few days in maximum length. North Korea would not invade the South because their aircover would be all but wiped out, so there would be no need for US ground force intervention.