Because it never stops at just stopping the enemy, the allies could have simply contained Nazi Germany but chose to invade. The US could have continued the war in the Pacific but chose to drop 2 atomic bombs.

Beating the enemy so thoroughly is the only way humans have ever been able to truly end a conflict of that scale, and in that situation if Europe is to beat Russia like that I don’t believe they’d hesitate to start launching nukes.

So we’re stuck in a situation where Europe either risk nuclear war, or an extended border skirmish that could last decades. Both would be devastating