The principles of self defense say that once you are no longer being attacked, any further aggression on your part is no longer defense. For example, you can use lethal force to protect yourself from a person attempting to cause you grievous harm, but once they stop attacking you and start retreating, if you chase after them and beat them or kill them, you’re no longer acting in self defense and are now committing a crime. By that same token, once the axis forces had been pushed back behind their own borders, invading them becomes an act of aggression rather than defense. Once they’re behind their own borders, fascist war mongering governments are an “internal affair” for the affected peoples to deal with.

Now you might argue that a declared war is no longer a situation where “non-interference” applies, but war can be declared unilaterally. So you might say that only the initial defenders have a right to engage in regime changes, but does that mean that the Ukrainian people have a right to overthrow the Russian government in response to the current war? Do the Palestinians have a right to overthrow the Israeli government? Do the Irish have a right to overthrow the British monarchy for their previous aggressions? Do the British have a right to overthrow the US government for the American Revolution?

Which ultimately is just a long way of getting back to my point that “non-interference” might be (and IMO is) a good default policy, it’s also an unrealistic one for all situations. At some point something about the current political landscape requires a nation to interfere in the “internal affairs” of another country. But that is a dangerous game that should never be the default.