Ukraine is in Europe. Nuking Ukraine means the wind will likely carry the fallout radiation into Russia, Belarus and Poland at the least.
I doesn't matter if its 1 nuke or a 100 nukes, irradiating NATO citizens is THE red line.
If Russia can nuke Europe without fear of reprisal, then NATO is defunct and a new nuclear arms race will probably start in Europe, with unforeseeable consequences.
So I would say yes, if Russia nukes a European capital with a multi-million pop, then nuclear war with Russia is the only course of action.