Having staging areas is a great advantage. The countries we were fighting were also engaged in two fronted wars, the Germans had us and the Russians, the Japanese had us and the Chinese. In the European theater we lost a vast number of merchant marines while simultaneously working to destroy the U-boat threat. We did have great Britain as a staging area, we also invaded North Africa first and used that as another front to attack Italy. In the Pacific, we mainly fought a holding war until the war in Europe was won. Thus we weren't actively engaged in a two front war the same way either as Germany or Japan. Then we island hopped using each conquered island as a staging area to attack the next island until we could fire bomb and later nuclear bomb Japan itself. Japan also failed to destroy the aircraft carriers of the Pacific fleet which allowed us to provide air support to the troops. We didn't invade Japan until they had surrendered. Against the US you could try to use Cuba as a staging area, but you have to go past Costa Rica and the US Virgin Islands, both of which would make fine air force bases to attack passing ships. Once you got here, the United States is a much larger land mass. Unlike Italy, France, and the rest of the European countries, Florida, South Carolina, or California wouldn't turn into an invading forces allies.

Anyway, arm chair strategies aside, I was just trying to make the point that personally owned firearms are a much, much smaller factor in a decision to invade the United States then say, geography.