American Identity went from National to Imperial.
It's like how Roman identity went from the property of real Romans to that of Non-Roman Others in the Empire.
Once identity becomes de-nationalized and then imperialized, the Core Population grows demoralized as what had once been special and unique to them becomes generic and diluted. And the new 'citizens' only care about the identity as a meal ticket.
Identity became like currency.
There was a time when currency was backed with precious metals, especially gold. But when gold standard was gone, any new amount could be printed at the whims of bankers or the state. Currency became easy to manipulate.
Likewise, Identity was based on blood. Once it was cut off from river of history and ethnicity, it was stamped with generic ink. So, anyone could easily become an American, and as Americanization is the globalist-imperium that sets the template for the world, every nation(except Israel with Passover exception) is 'Americanized', which means its identity must also go from National(unique and special) to Imperial(generic and mercenary).
So, there is no longer any difference between American, British, German, French, Italian, Spanish, Irish, Brazilian, and etc. 'Diversity' and Elite-Minority-Supremacism define their New National Identity, which is anything but truly national. They are interchangeably Global. France is turning into a nation of 'Americans' who speak French.
So, just as anyone can easily become American, a newly arrived African with no roots in France is just as French as a Frenchmen with racial & cultural roots going back many centuries, and a newly arrived Turk or Syrian with no roots in Germany is just as German as a German with racial & cultural roots in Germany since time immemorial.
Americanism is no longer just a national idea that applies only to America but the ONE AND ONLY IDEAL FOR ALL NATIONS(with the exception of Israel that must remain Jewish-majority and Jewish-ruled).
The reason why imperialists hate nationalism is two-fold.
Imperialists are filled with hubris, and their own nation is no longer big enough for their ambitions and pretensions. So, whereas Bismarck was content with German nationalism, Kaiser Wilhelm and later Hitler wanted world empires. And in striving for empire over nation, they brought ruination upon Germany.
In striving for empire, imperialists neglect the sober interests of their own nation and drive their nations to huge risks... like Japan did in WWII that led to defeat and destruction.
Today, we have Merkel and her Empire of Compassion that is based on Moral Hubris. That too is destroying Germany. And Sweden's pretensions as a 'Moral Super Power', a Empire of Political Correctness, is leading it to Swedish Suicide or Swecide.
Another reason why imperialists hate nationalism is it undermines the legitimacy and power of the empire. After all, imperialists have majority power only in their homeland. Outside the empire, they are alien minority elites over OTHER peoples. If nationalism were the norm worldwide, native masses will rise up and push out the elite minority imperial overlords. So, the ONLY way the imperialists can justify their power over Other Peoples is to make the latter feel as part of a Great Empire that transcends tribal or national interests.
This is what the British did. Consider how the Brits used to rule over India, Malaysia, Kenya, Uganda, Rhodesia, Palestine, etc. (And the Japanese sold their regional globalist-imperialism as a radiant project of the Great East Asian Co-Prosperity Sphere.)
By rules of nationalism, each of those peoples would have struggled for national independence & sovereignty, sought to drive out the British overlords, and have their own nation. Indians would have demanded India for Indians, Malaysians would have demanded Malaysia for Malaysians, Kenyans would have demanded Kenya for Kenyans, and etc. That would have meant end of empire.
So, the British Imperialists sought to convince their non-British subjects that it was glorious and gratifying to be part of this grand wondrous empire under the loving care of the gentle queen. Also, the British imperialist elites appealed to class/caste as a binding principle among various peoples in the empire, i.e. however different they may be in race or culture, the non-white (comprador) elites and British elites belonged to a shared community of superiority and privilege.
If nationalism emphasized the special bond between national elites and national masses, imperialism urged subject elites to collaborate with the ruling imperial elites while neglecting their own peoples. Under British Imperialism, the great contradiction lay in British elites' representation & defense of British masses while pressuring non-British elites to forgo their patriotic responsibilities to their own peoples. Only the British elites could be both nationalist and imperialist whereas subject elites in the empire had to operate only in the imperialist mode of obeying British mandates.
Today, as the Anglosphere World is the subject domain of Jewish globalists, the ONLY people who have the privilege of being both nationalist and imperialist are the Zionists. Only Jews can have both powerful nationalism in Israel while, at the same time, demanding that gentile elites cut their ties to gentile masses and serve Jewish Globalists elites instead. If you disobey, you become ostracized, isolated, and sanctioned by Russia under Putin and Hungary under Orban. (Jews saw right through the British BS, and early Zionists in Palestine had no desire to serve as comprador elites of the British Empire. In time, they used terror to drive out the British, and then used war to ethnically expel the Arabs to create a Jewish nation-state. Ironically, even though Israel was an imperial creation, it was the first non-Western nation to 'liberate' itself from European imperialism after WWII. Zionists, being imperialists themselves, understood the guile of Western Imperialism better than most.)
Today, Jews think and act not unlike the British Imperialists of old Jews are only a majority in Israel. While Jews want a Jewish state in Israel, their power and influence are precarious in all other nations. Why? Because there are few or no Jews in most gentile nations. Nationalism is a barrier to Jewish penetration and takeover. So, in order for Jews to take over the entire world(by using American neo-imperialism controlled by Jewish elites), Jews must convince the entire world to 'Americanize' as America goes from a white nation to a non-white one. So, America is to turn non-white, and Europe is to 'Americanize'. And then, all other gentile nations must dilute their national identities and formulate a new shared globo-imperial identity based on Americanism that says anyone can become anyone almost overnight based on 'rule of law' as devised by Jewish globalist lawyers. And the new shared neo-religion for all peoples must be worship of Homomania since Anno Sodomini.