But if this were Hawaii and we were invaded, the invaders would know exactly who has what guns and where they most likely live. Compared to other states that have no registration.
That scenario could potentially happen, if we were living in the 20th century, with the US fighting in a war with a military peer.
IMO, assuming no use of nuclear weapons, a war between military peers in the 21st century will begin with economic embargoes, tariffs, sanctions, and etc. to weaken the military financial base of the opponent, then fought with missiles and probably EMP type weapons to neutralize military and civilian infrastructure. The resulting SHTF situation and widespread starvation, disease, rapes, murders, and killings on the streets would do sufficent damage, compelling the government to bring a quick end to the war.
The use of ground troops would be used by the victor in the end to quell or intimidate the small pockets of unarmed/armed resistance and serve as an occupying force, as currently seen with US military forces illegally occupying Syria and Hawaii.