Here's a not entirely implausible scenario:

1. Putin 'wins' in Ukraine.
2. Trump wins or 'wins' the election.
3. President Trump (again) says he won't protect European NATO allies.
4. NATO is effectively dead. The US nuclear umbrella no longer protects Europe.
5. An emboldened Putin, engages in constant provocations and distractions.
6. Some EU members are forced to drastically increase defense spending. Others come to the conclusion that acquiring a nuclear deterrent is now a necessity. Populist Putin verstehers win elections in some European countries, in part thanks to support from an emboldened Kremlin, promising to reduce defense spending. The EU project effectively dies. Europe is divided.
7. A large part of the EU now kowtows to China.
8. Because Europe is weakened, Putin continues to cause provocations and a miscalculation is made OR Russia deliberately invades the Suwalki gap (which is already indefensible now), thereby connect Kaliningrad with Russia. Putin's allies have regularly suggested that this is part of the plan.
9. Europe is now at war with Russia. The US does not intervene, because of Trump or his isolationist successor.
10. Europe is weakened from decades of underinvestment, weapons caches are depleted having supported Ukraine, Europe hasn't had enough time (or will) to properly re-arm, Europe is divided, it is not clear who will protect who.
11. Certain European countries feel existentially threatened. They have no conventional military response to Russian aggression. Russia is making steady gains.
12. Some European countries still have a non-conventional military response.
13. A nuclear war starts in Europe.
14. Most importantly for American readers, this significantly raises fuel prices.
15. Half the population blames a long dead Biden and soy based diets.