Chris in London said:
For the hard of understanding, if the Russians do violate Ukraine's territory and the Ukrainian's do ask the UK/US for assistance (as they are entitled to do) we have two choices:
(1) comply with our treaty obligations and provide military assistance to the Ukraine.
(2) refuse to comply with our treaty obligations.
Option (1) may be a disaster waiting to happen in military terms but Option (2) is a disaster in diplomatic terms.
What worth will any treaty obligation the UK signs up to have if we renege on a commitment to Ukraine? What status would Britain have in the world if it makes promises that it doesn't deliver on when called to do so?
When you are between a rock and a hard place, bad things can happen.
How did it go in Syria when we were asked for help (as they were entitled to do)? How did it go in Egypt when we demanded that democratic elections take place and when they did and the Muslim Brootherhood got in we suddenly did not like that version of democracy?
The UK, USA and Russia will not rock the boat in any way that could see conflict directly between those countries.