How could a Europe-only response to a Russian nuclear attack on Ukraine ever be adequate?
If we wanted to avoid a direct military response, we would have to declare Russia a terrorist state, break off diplomatic relations, impose an absolute trade embargo (NOTHING sold to or bought from Russia) and send lots of weapons to Ukraine. This couldn’t be done without the US because the trade embargo would have to be universal (or nearly universal) - including China and India - and only the US is strong enough to force China and India to boycott Russia. Only the US has enough weapons to substantially strenghthen the Ukrainian military.
If wanted to respond conventionally, only the US has air assetts strong enough to crush the Russian military in Ukraine from the air.
The only option the Europeans have is using a tactical nuke. This would - in my opinion - be the riskiest option.
All of Europe has enough assets to defeat Russia.
The US isn’t Europe’s police.