General Discussion

if that ideology is expansionist, it will by necessity become an existential threat to other ideologies.

This is it right here, and it's why the world didn't really care what a threat Putin was until he fully invaded Ukraine, or about the Uyghur genocide, or the treatment of North Koreans, or Gazans, or Kurds, and on and on - the people responsible for those are, quite deliberately, trying to keep their actions as much as they can within their own territory to avoid giving any other country reason to physically get involved (though they are deeply involved otherwise in the supports, financial and otherwise, that they provide, often to both sides of a conflict).

You can say it became a threat to their power, but is that not also a threat to their ideology? Perhaps I’m using an overly broad conception of ideology here.

No, you're right here too, the "good guys" aren't that good at all, and are only acting to defend their own interests - hoarding wealth and power, and maintaining the systems that enable them to do that at the expense of everyone else.
In actuality capitalistic imperialism specifically has and continues to claim more lives than probably all of the others combined (not only directly through war and invasion, but also through hunger poverty and preventable diseases which are a requirement and an inevitable result of artificial scarcity and commodification of what are basic human rights for profit), but history is written by the victors, and we are indoctrinated from birth in to false ideas of what humanity, freedom, and justice actually mean, in service of keeping us in line.