History is moving on, and now even the nation-State is not large enough to compete. States unite in ever-larger units, such as the European Union. I have no doubt that by the end of this century, there will be in place an effective world government, turning the entire world effectively into one state. (If some extra-terrestrials threaten this world, it will help.)
So how does the separation into smaller and smaller states fit this trend? Simply, if the state of Spain is not necessary anymore for economic and military purposes and its central functions are moving from Madrid to Brussels, why shouldn't the Catalans and the Basques secede and join the Union under their own flags? Look at Yugoslavia, look even at the Soviet Union. Germany is the great exception but it is quite large by itself.
The two processes are not contradictory, they complement each other.
The idiotic Brexit is ahistorical. But if the Scots and the Welsh want to separate from England, they will succeed.
I have great respect for the power of nationalism. In our era, it has proved, to be stronger than religion, communism or any other creed. It is strongest when it combines with religion, as in the Arab world. So the nationalism of small peoples will gain satisfaction in football games, while the real business will be conducted elsewhere.
AT THIS very moment, the Israeli Parliament, the Knesset, is busy with enacting a new law, called the Nation Law, which is intended to make clear that the Jewishness of the Jewish State takes precedence over democracy and human rights.
Israel has no constitution, but until now it was assumed that Israel was equally "Jewish" and "democratic". The new law is about to abolish that notion.
As usual, we are one or two centuries behind world history.