First of all I would ask what is this win scenario you envision?
Is it: 1)Russia takes over all of Ukraine and installs a puppet government, subservient to the Kremlin? That option is long gone.
2) Russia expends thousands of tanks and young men to capture some of East Ukraine? They would face an insurgency and ongoing battles for control, not like ISIS in Afghanistan, more like the trench warfare from WWI or urban fighting from WWII.
There is no win scenario for Russia that will account for the loss of: 1. soldiers
2. Military equipment (destroyed/captured/spent)
3. The mythos of the Russian Armed forces being destroyed
4. Russia's Economy
5. The young talent fleeing the country (brain drain)
All of these losses will take decades to recover from and the Russian people will suffer long and hard, they already had a falling population, this will accelerate the problem.
I think a better question is: what if this conflict - and the accompanying energy and food shortages - are here to stay for the foreseeable future.
I just checked http://stopgas.org/en/, it's still turning over.
The west's approach in that circumstance would probably be to leave the sanctions in place indefinitely.
The only real surprise was how fast the Taliban was able to take the country back, not that they were.
To answer the rest of your question. I think it's pretty clear that the response would be mostly condemnation and sanctions. There's no appetite for direct military conflict between NATO members and Russia, at least on the NATO side, so what are you going to do?
I'm sure the US would supply arms to insurgents, but that's something the US does a lot.
I'd put my money on the perceived weakness and incompetence being shown in one of the larger western countries that enabled all of this to happen.
It seems that someone likes creating wars all over the world and is doing nothing to prevent them.
As a result of failing to prevent it, ultimately in the end everyone loses.