The most dedicated critics of nationalism and national identity have been Marxists, no surprise there. For Marxists, nationalism is merely a bourgeois means of mobilizing the masses in favor of the existing political and socioeconomic system and turning righteous anger against people of other identities, both within and without a particular country. Perhaps the strongest denunciation of nationalism came from a British Marxist historian, Eric Hobsbawm. It is true he was for most of his life a Communist, but some of his observations nevertheless remain insightful.
It is my opinion that the current controversies in Britain about the alleged anti-Semitism of the Labour Party are largely the result of anti-nationalism. Zionism is a Jewish nationalism which developed in defense of Jews attacked by the ethnic nationalism of the nations in which they lived. Zionism aimed not only to restore the Jewish sense of nationalism -- something in fact they invented in the Hebrew Bible -- but also to make Jews normal like other peoples, with their own nation, their own free identity -- one not based on gentile sympathies or generosity but on their own power and rights. The Labour Party has always been anti-Zionist; it opposed the creation of Israel and the entry of Jewish refugees and other into Palestine during WWII and thereafter. In this the Labour Party is following Marxist principles. Jews in Britain, not a very large population compared to France for example, could vote Tory, but Tories are noted for their genteel anti-Semitism -- not too extreme, but still there. What are Jews in Britain to do? I suggest they need to emigrate to Israel or to the one country which is not founded on Blut und Boden ethnic identity but on principles -- namely the United State of America. We should be proud to be such a nation, and the Trumpites don't like it one bit.