Liberals want people to forget their cultural and ethnic bonds, or at least ignore those bonds in making decisions. By this token, people should also forget about their families, which indeed happens as family ties deteriorate in post-modern societies. One’s cultural background determines one’s self in the sense that it affects his free choice: in every situation, the choice remains free, but it is influenced by background.
Large states have lost their major advantages over small ones. The WTO diminished customs duties and reduced the importance of tariff-free trade over large territories, which was previously only available in mega-states. Terrorism has undercut their security guarantees.
It would take a shakedown to break mega-states into culturally homogenous entities. A slow process would be digested by the citizenry and would not lead to major changes. The deterioration of the economy under increased redistribution won’t do. Increased autonomy is hardly possible in the face of nationwide customs policy and regulation of local traditions through federal courts.
Major security problems will do it. Warring states did not use poison gas in WWII for fear of mutual escalation, which would have prevented both sides from fighting. Likewise, states are not likely to use nuclear weapons even in a worst-case scenario. At most, nuclear explosions will be used to create no-pass zones for attacking armies. But terrorists will use them. Locales, accordingly, will start proclaiming themselves neutral, sort of like the city of San Francisco banning voluntary enlistment in the US Army.
This scenario can be hindered by development of radiation detectors capable of identifying a nuclear bomb even on a ship approaching a port.

Large states are not needed when any locale can install SAM batteries and nations grow affluent and unwarlike.

Large states are not needed