Your argument doesn't make much sense.
You started by talking about the actions of the U.S. government, but then immediately switched to insulting an individual for being American. If your issue is with the U.S. government, its military, or its foreign policy, then criticize those. Saying "fuck off American" isn't a criticism of a government it's an insult directed at someone because of their nationality.
You also keep acting like disagreeing with your generalization means someone is defending U.S. imperialism or war crimes. That's a false dilemma. Someone can think the U.S. has done horrific things and still think it's wrong to judge ordinary Americans based on where they were born.
And your historical argument isn't unique to the United States either. If your standard is "this country has committed atrocities, therefore its people deserve hostility," then you'd have to apply that to nearly every major power in history.
The British Empire colonized huge parts of the world, committed atrocities during colonial rule, and exploited countless populations. France carried out brutal colonial campaigns in places like Algeria. Belgium's rule in the Congo led to millions of deaths through forced labor and violence. Imperial Japan committed massacres, forced labor, and sexual slavery across Asia. Nazi Germany carried out the Holocaust and started a world war. The Soviet Union committed mass political repression, forced deportations, and famines. The Russian Empire and later Russia have waged wars and committed abuses against neighboring peoples. China has a long history of imperial expansion, and its modern government has been accused of serious human rights abuses. The Ottoman Empire committed mass atrocities, including against Armenians. Spain and Portugal built colonial empires through conquest, enslavement, and exploitation. Even states in the ancient world expanded through warfare, conquest, and subjugation.
None of that means every British, French, Belgian, Japanese, German, Russian, Chinese, Spanish, Portuguese, or Turkish person is personally responsible for those actions. So why would that logic suddenly apply only to Americans?
You also said America "isn't a real country," which isn't a serious historical or political argument. Whether someone likes the United States or not, it's internationally recognized as a sovereign state. Calling it "not real" doesn't strengthen your point.
Finally, telling someone to "read theory" isn't a substitute for responding to what they actually said. They didn't defend U.S. foreign policy. They said xenophobia isn't okay. Instead of addressing that, you insulted them because of their nationality.
Criticize governments. Criticize militaries. Criticize foreign policy. Criticize imperialism. But blaming random people for the actions of a state simply because they were born there is prejudice, and no amount of political theory changes that.