I dont know the official rules, but I think that this kind of attacks should be treated as Surprise attacks. A neutral nation or unit is always taken by surprise when attacked for the first time. A&A Pacific 1999 ed had a Surprise rule, that said all defending units roll a 1 for hit during the first round of combat, when surprised. Of course this was meant for the Pearl attack. If all battleships and fighters had defended at full value, this attack would be impossible to pull off.
But, funny thing was, that Surprise rule had strong effect on expensive units like ships and aircrafts, and lesser effects on infantry. That is exactly how it was in the real war too. The greatest surprise comes when you attack from the sea or the air, against other naval and air units. And lesser effect when attacking from adjacent territory against infantry. Since a neutral nation is not in war, the minefields are not activated, so enemy ships can easily sail into the harbor. That was true for Norway, Denmark, Pearl, Singapore and Hong Kong. Also land attack can make some surprise, like Germany attacking Polen, France and Russia. But since it takes time to drive through an infantry trench with millions of men, the surprise will soon be gone. What makes this rule true, is the Russian attack on Finland. That was not a surprise attack, since Russia declared war on Finland before they combat moved in, the fins were ready and defended on max value.
I think the rule should go like
When attacking neutral units, the neutral units defend on 1 for the first round of combat. Maybe it should be the first turn, playtest will show. But most important, the neutral units will return fire, just not at max value.