I honestly think that at this point, Ukraine is already in the hands of Russia. While finally it all comes down to military might, Ukraine currently owes Russia quite a bit of money (as well as debts for gas and oil). It's all a huge clusterfuck.
As to the US helping out Ukraine, I think that is very limited as well. The US will not directly interfere (I believe that is what they mean by 'leathal aid' but not 100% sure).
If this were a game of chess - and I apologize in advance for making the comparison - a full-scale Russian invasion into the rest of Europe will only depend on how the UN, US and UK act towards the rest of the neighbouring countries from Eastern Europe. I have to say I marvel at that worthless piece of shit David Cameron and his obsession with islamist terrorism, while Ukraine is being invaded. I somehow think that, in his mind, islamist terrorism is a higher priority because it can happen at any moment in his front lawn. Which only shows the extent of the UN's solidarity and the value of UN treaties when it comes down to acting.
I feel that the US is going to do much more than the UK in regards to Russia's agression. But I still think Ukraine is already lost & forgotten.