Now: wich countries did have open war in the 20th century, wich both owned nukes?
None.

Im not talking about third-country placeholder-wars between the US and Russia (such as Vietnam), - but open war between nuklear powers.

The whole reason for a country to optain nukes is not to use them, but
to get shielded from attacks. (such as Iran and North Korea)
or even better: Pakistan and India.

Once they own nukes, they have a certain savety from open war with
big military powers.

The attack on Irak for example is a good indication, that the US did not believe they really own workable nukes...