The significant factor in these cases of banning (or destroying) other nations wasn't language, race, or culture, but religion. Individuals of any origin were, and still are, welcome to convert to Judaism. (Consider how many "mixed marriages" there were, including some of Jesus's ancestors.)

As the quotation indicates, the crucial factor was the concern that these people would cause false religion to corrupt Israelite society.

Compare this with today's world, where some people object to the way immigrants are changing society. That position assumes that society is, in absolute terms, the ideal society and shouldn't be changed.

That's not a justifiable view. Our societies are generally the way they are now, because of the effects of immigration and the integration of foreign culture, not despite them.

But in the case of Israel, this is a justifiable view, at least in terms of God's laws. Israel was chosen as God's ideal society; its purpose was to set an example to the rest of the world. For over a thousand years Israelite society repeatedly prospered and declined, all in direct correlation to how well its king and the people followed God's rules.

Whenever foreign religion entered and influenced Israelite society, the nation experienced setbacks (wars, poverty, etc.). Whenever the King turned to God and cleaned up society, the nation experienced good times (peace, affluence, etc.).

This up and down cycle was in fact Israel's very purpose. The Pharisees understood this, and that is why they enhanced the biblical laws (if one follows an exaggerated law, it leaves no doubt that the underlying law is being followed), and why they policed society the way they did.

So no, the Tanakh (OT) does not support ethno-nationalism in general, but yes it did support it for the Israelites.

I think the answers so far give solid answers to your question. In short, the answer is "No. The Bible does not support Ethno-Nationalism because such a distinction over what we think of as ethnicity today would be unlikely at the time the Bible was written.

I would pose the idea that those to whom Moses was declaring these rules probably did not have much exposure to different "races" like we have today. Since travel was difficult, most journeys were too short to bring people in contact with other races. The idea that God would have demanded ethno-nationalism of the Hebrews when it was difficult enough just to keep them monotheistic and adherent to the Law of Moses seems a bit of a stretch. I seriously doubt your friend's interpretation.

If it was important to God, and if He felt it was important to Israel to stay "pure" in terms of race, there would probably be a commandment in Exodus or Deuteronomy that says "Thou Shalt Not..." and the rest would be something about intermingling of races or colors or something. This is not the case. What we DO have is a clear commandment that "Thou shalt love and fear only the Lord your God and have no other gods before Him." (Which isn't exactly verbatim, but if you want to know how it is stated in various translations look up the First Commandment on BibleGateway.com).

The First Commandment serves as context for both of the very good answers above because both of the answers correctly indicate that race is not a primary concern of God, but whether the beings He created in His own image would continue on a path that would ultimately lead them back to salvation and reconciliation has always been God's concern (aka the Kingdom of Heaven).

Here's a question: Does your friend believe that there is a hierarchy to "Ethno-Nationalism"? If so, then he must recognize that there is nothing in scripture that tells us which race is the "highest" or closest to God (unless we consider Jewishness to be a distinct ethno-nationalist category). As for religious ethnicity, if your friend is correct, then from the whole of the New Testament it is pretty clear that the Followers of Christ are only blessed if they voluntarily choose to put themselves on the bottom of any Ethno-Nationalist hierarchy. Or is your friend merely saying that God wants races to keep to themselves, entirely separate but equal? If that was the case, why would Jesus have bothered to command his disciples to preach the Good News to the ends of the Earth?

The notion that race matters anything to God does not hold water when the entirety of scripture is considered. The idea of ethno-nationalism in the first place is just eugenics by another name, and is probably only a consideration for people who were borne since the height of the Roman Empire (we need decent roads before we can reach other races). Thus, the Old Testament probably doesn't even apply to the concept. If something doesn't pass the "Does it agree with the entire Bible" test, it is pretty certain that it's just verses out of context intended to justify a narrow and selfish interpretation of some tiny portion of scripture.

For The Ancient Israelites, living under the Old Covenant, yes. For Christians, no.

The book of Deuteronomy is one of the books of the Bible written by Moses, describing the terms and conditions of the Old Covenant, which was an agreement between the Israelite people and God. One of the terms of that agreement, as your friend noted, was a prohibition against intermarriage, and that formed the basis of an ethno-nationalist state.

However, when Jesus came to Earth, he fulfilled the terms of the Old Covenant, and established a New Covenant between his followers and God. The terms of the New Covenant aren't ethno-nationalist in nature; as Paul puts it:

So in Christ Jesus you are all children of God through faith, for all of you who were baptized into Christ have clothed yourselves with Christ. There is neither Jew nor Gentile, neither slave nor free, nor is there male and female, for you are all one in Christ Jesus. If you belong to Christ, then you are Abraham’s seed, and heirs according to the promise.

Galatians 3:26-29

That said, God's nature has never changed, even if the terms and conditions of his agreements with humanity have, and he wouldn't have required the ancient Israelites to adhere to any contract terms that he found morally wrong. From this, we can then make the inference that, at the very least, God does not find the existence of ethnonationalist states wrong - He may or may not support them, in general, and they may or may not be the ideal form of government in God's eyes, but at the very least He doesn't oppose them.