The current round of fighting.
There's still quite a bit of Europe that Russia wants to plunder for resour... I mean... influence. All of Ukraine, Poland, the Baltics, the Balkans - anywhere that used to be a part of the Warsaw Pact - is a target.
Appeasing Putin now will likely see another military action in the next decade, especially if the US significantly reduces or totally ends its commitment to NATO.
To be fair, the US has done nothing but harp on its allies in the organization to increase their g_ddamn defense spending for the better part of three decades now, and only some of them have truly taken it seriously since 2022. Even if spending levels are increased now, it won't have enough of an effect to see battlefield dividends for several years, if not at least a decade. The end result is that the US will be the security backstop for a Europe that has taken American willingness to get into a possible thermonuclear war for granted, in the face of more Russian incursion, no matter how poorly handled that incursion might be.
The UK and France also have nukes. I'm not sure about the UK, but France at least would shield NATO and EU with the nuclear umbrella even if the US leaves NATO. Finland, Romania and Poland might want to ask France if it minds sharing.. ideally before the alliance gets so weak that Putin decides to test our resolve.