Why does nationalism still hold us enslaved in the twenty-first century?

The new Roman Empire that is Europe, should have steadied up a little, and perhaps the Russian Federation should have equalised things with their old Soviet republics neighbours, so such a thing as the war in Ukraine was impossible. After all, each people should be able to self-determine, but the old relationships between peoples should be done in a peaceful co-dependence, interdependence, that stops the dreadful chance of war.

We had wars of nationalism the whole of the Twentieth century, surely we are past that!