Although written in 1945 and reflecting a world still embroiled in World War II, Orwell's essay "Notes on Nationalism" is relevant for our times because nationalism is largely understood to be on the rise again worldwide. Therefore, many of the issues Orwell raises still trouble us today.
Orwell states that a nationalist is not necessarily one blindly devoted to one's country, though a nationalist can be that, but one blindly devoted to any cause—be it communism; Catholicism; any form of what we might today call identity politics (Orwell names Celtic nationalism and Zionism); or a stance that can be defined as being against some group, such as Jews or the British upper class.
Nationalism, in whatever guise, has three main attributes: obsession, instability, and indifference to reality. The nationalist is obsessed with the correctness of his cause, "unshakeably certain of being in the right." Instability comes because nationalists can change their cause, but not the fervor, of their convictions almost instantly. As Orwell puts it,
A country or other unit which has been worshipped for years may suddenly become detestable, and some other object of affection may take its place with almost no interval.
Finally, because the nationalist is so sure that his cause is right, he will distort or ignore any fact in order to promote his own side. This not only means ignoring inconvenient facts or creating what we might today term "fake news," it means that any atrocity one's own side commits is acceptable.
Nationalism is obsessed with winning. Orwell writes,
A nationalist is one who thinks solely, or mainly, in terms of competitive prestige. He may be a positive or a negative nationalist—that is, he may use his mental energy either in boosting or in denigrating—but at any rate his thoughts always turn on victories, defeats, triumphs and humiliations.
Orwell's final point is that all of us are susceptible to nationalist thinking, in which we see "our" side as all good and the "enemy" as all bad—and this can turn any of us into moral monsters. As he states,
the most fair-minded and sweet-tempered person may suddenly be transformed into a vicious partisan, anxious only to "score" over his adversary and indifferent as to how many lies he tells or how many logical errors he commits in doing so.
We must therefore accept our own tendencies to nationalism and be on guard against them.
In our world of fierce loyalties to ideology, nation, or tribe, we can easily see the relevance of Orwell's warnings in this essay. It is too easy to completely justify "our" side's point of view while harshly condemning the other side as guilty of the worst evil for doing exactly the same things. It is also worth noting that Orwell fleshed out many of these ideas further in his novel 1984, which was written only a few years after this essay.