Every so often I get downvoted for something I consider completely noncontroversial, but with no comments to say what the objections are.
The general term for the kind of ideology Orwell is talking about is collectivism, and the epistemological error is methodological collectivism. Clearly nationalism is just not the right word, because he is not just talking about nations. Collectivism more accurately captures what he is talking about. If anyone has a more precise and accurate term, feel free to share.
I've more often seen collectivism as an opposite to individualism: that is, a desire or tendency to work together in groups or to focus on the welfare of a group more than that of individuals in it, or in some cases a focus on preventing harm over maximizing freedom. Tribalism is better because it doesn't have any other meaning. Also, the tendency Orwell is talking about probably evolved during conflicts among hunter-gatherer tribes, but I have no citation for that.
I have found this most wonderful (if fairly lengthy) article, and thought you would enjoy having it brought to your attention.
It is so good, I think we should include it among the references in the "Politics is the Mind Killer" wiki page. But, before that, I submit it to you, and ask you: is there anything in this article that would warrant its exclusion from this site? I mean, besides the fact that it is about politics, and written by a notorious Social-democrat (And is that in itself grounds for exclusion?).