Are you sure? Jews and Samaritans considered each other enemies to the extent that they would not talk to each other.
Jesus deliberately made a very sharp point about such hostility in the parable of the good Samaritan. They were enemies until the Samaritan made the difference.
Jesus was consistently gracious to the occupying Romans too.
I'll say again, it's a proper function of a nation to protect its citizens.
Nationalism goes further than that: it asks that we put loyalty to the nation above other interests. I cannot do that as I believe that God has created people of all nations, and loves with an all-surpassing love. If I were to put my nation above others, I'd be sinning against the vast majority of the world which is not British.
Jews and Samaritans were divided on religious grounds, not nationalistic. (1 Kings 13:32, 16:29). Such a division was from God. (1 Kings 12:24)
God has always divided. Division of man curbs wickedness among men. The more united man is, the more wicked he becomes. Consider the flood. No divisions of nations prior to. After the flood God began dividing man. (Gen. 10:5, 10;20, 10:31) God told Noah and his sons to move out into the earth and replenish it. But their descendants instead united together at Babel and refused to be scattered. (Gen. 11:4) And we know how that turned out.
God has divided the land masses through the use of oceans. (Gen. 10:25) And He has divided the nations through race and languages. (10:31-32) All for the intent to curb or slow down the wickedness of man.
And it works. But satan is busy. He puts much effort in uniting mankind. Much effort in building the bridges between these divisions God has made. And so now days, the 'unity of man' is seen as such a glorious thing. Which it would be if man were united under God, but he doesn't. He is just united man, with evil multiplying in the earth till it comes to the place again where God will bring about judgement upon man on the earth.
So, nationalism is a good thing. It helps stop the works of satan. That doesn't mean every nation is following God. Some of them, satan has in control. But it restricts satan controlling everyone of them. And of course this is his, satan's, effort at uniting the nations today.