The decision to sacrifice Ukraine to Russia has already been taken since last year
I need to clarify certain facts. Here is what David Cameron said in July 2014:
"Britain is not going to start World War Three over Russia’s involvement in Ukraine"
That is fine and was a legitimate statement/concern. Unless there had been an official treaty between Ukraine, the UK, US and Russia, stating:
"The Russian Federation, the United Kingdom of Great Britain and Northern Ireland and the United States of America reaffirm their commitment to seek immediate United Nations Security Council action to provide assistance to Ukraine, as a non-nuclear-weapon State party to the Treaty on the Non-Proliferation of Nuclear Weapons, if Ukraine should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used."
Actual paragraph of the signed treaty, back in 1994.
So the short story is that in exchange for Ukraine giving up their nuclear weapons, they would get some degree of protection from the UN.
Let's go back to what led to Ukraine having to give up nuclear weapons in the first place:
"In an effort to put pressure on Ukraine to give up the nuclear weapons left on its soil by the Soviet Union, the Clinton administration has rebuffed Ukraine's request for a meeting between its prime minister and President Clinton or Vice President Al Gore, senior administration officials say. The officials say the rebuff is part of an effort to get the Ukrainians to fulfill their promises to ratify the arms-reduction treaty and give up the nuclear weapons. [...] Ukraine's position is crucial because Moscow and Washington have said they will not begin to carry out the arms-reduction treaty until Kiev ratifies the accord and joins the nuclear Non-Proliferation Treaty, which is intended to stop the spread of nuclear weapons."
(April, 1993 from this article)
Here is my opinion: politics is probably like a game of chess but should we truly start with the presumption that everything is really bullshit, that promises (and even signed treaties between nations) have no value anymore? That there is no solidarity between friendly nations but only personal agenda and profit-oriented last-moment decisions? I personally think Ukraine should have been more ruthless with their nuclear weapons, at least they should have fought to keep part of those 2000 warheads. They probably should have lied and fight in a dirty manner but then again doing so would have meant isolation from the US and the UK. Instead Ukraine chose to comply and go with the legitimate way. And the result has been: annexation of Crimea in March 2014, annexation of eastern Ukraine in August 2014 and - my opinion, but probably more than just an opinion at this point - an inevitable invasion of Ukraine in 2015.