The thought has occurred to me that the United Nations should have a 'peacekeeping' force present in Ukraine behind the frontlines, ensuring that, should the Ukrainians lose ground and need to retreat, the Ruzzians cannot advance further into Ukraine, lest they encounter UN forces protecting the region.
The UN coalition would not engage with Ruzzian troops unless fired upon themselves. Not much point trying to 'take over Ukraine' when you're isolated to the only part you've invaded.