[cvejwwrf] Immune from invasion

In the olden days, if one country thought another country was weak, they would invade (and possibly genocide).  (Inspired by Lebensraum.)  This created incentive for countries to maintain some sort of a reasonably functioning, economically productive state.  (Unlike the collapse of the western Roman empire.)

Nowadays, if a country can field a nuclear arsenal, they can retaliate against invasion with such devastating force that no one will dare invade.  Therefore, there is no longer as much incentive to maintain a reasonably functioning state.

Is this logic true?  Are nuclear weapons unique and unprecedented in the history of military warfare in their ability to provide immunity from invasion?

Maintaining a nuclear arsenal does require a reasonably functioning state.

If a state is functioning poorly, even without invasion it may fall to internal forces of regime change, so the incentive to maintain a reasonably functioning state still exists even in a nuclear armed power.  (E.g., regime change in Soviet Union.)  Civil war could have external countries supporting the rebels.  Is an external country supporting and winning a rebellion functionally equivalent to invasion?  One could imagine the installed puppet government committing acts like genocide to benefit that external country that supported the rebellion.

If a nuclear armed government is facing internal rebellion supported by an external country, will they use their nuclear weapons against the external country?  Should they?

Is the incentive to maintain a reasonably functioning state even good?  One could imagine a state using its nuclear umbrella to allow its citizenry to live peaceful and happy, albeit technologically backward and less economically productive, lives.  In contrast, in the past, a "reasonably functioning state" probably meant exactly one thing, "able to field an army", which is not necessarily good for the country or for human civilization as a whole.

Military alliances between nuclear and non-nuclear armed powers also cause weird effects.  The non-nuclear ally must accede to the desires of the nuclear ally or risk being expelled from the alliance.  Meanwhile, the non-nuclear ally gains the immunity from invasion accorded to nuclear armed powers.