Russia has a lot more troops than Ukraine. I doubt that without the influx of NATO troops Ukraine will ever get the upper hand in this war. But if nukes ever come into play then all sorts of avenues for success open up for the Ukrainians. My guess is a small suitcase nuke will be set off in Moscow near the Russian parliment and that will either cause a cessation of hostilities or Russia will get pissed and end the war very quickly, with a lot of civilians in Ukraine bearing the brunt of the backlash.