Did the interim government have the right to expect a more robust response from Western countries in response to Russia's move to effectively separate the Crimea from its control? Leaving aside the question as to whether military force ought to be used--which sets up an entirely different set of questions for the ethical policymaker--we must still assess whether or not the new administration in Kyiv took policy steps with the understanding that the West would take measures to help mitigate a predictable negative Russian response.
Much attention has focused on the so-called Budapest Memorandum. As part of the process to get Ukraine to give up Soviet nuclear weapons and other components it had inherited after the collapse of the Soviet Union, a long and complicated set of negotiations took place. Some in Ukraine wanted to keep nuclear weapons as a hedge against the possibility that Russia, in the future, would use its superior power to pressure Ukraine or even reincorporate it into a new union. The government of Leonid Kravchuk was persuaded to part with the weapons, in part by promises of massive Western foreign aid, and guarantees that Ukraine's independence and territorial integrity would be protected. The Budapest Memorandum, signed in December 1994, was a promise and pledge of the United States, the United Kingdom and Russia to secure Ukraine's borders and to protect Ukraine against attack particularly by a nuclear power.
The problem, however, is that like many other "understandings" reached during the late Cold War and early post Cold War period, this was not set up as a formal treaty. So,
This promise was an important incentive in getting Ukraine to give up its nuclear weapons. However, it is not akin to the treaty which created Belgium and which guaranteed its security--the stated reason for Britain to declare war on Germany in 1914 after the German army violated the Belgian frontier. It is, as the title suggests, a memorandum. It was never ratified by the Senate and it offers no security guarantees whatsoever to Ukraine.
Ukraine has also suffered from the tendency of the United States and other Western countries to describe Ukraine as a "partner"--"to suggest a quasi-alliance relationship with a country without any formal treaty guarantees." This deliberately-ambiguous term could be spun as needed, to depend on the ear of the beholder. But it has also meant that the "clear, bright shining line between an ally with a Senate-confirmed mutual defense treaty and concrete contingency planning for security and a "partner" who received U.S. security assistance funds and basked in presidential speeches and non-binding Congressional resolutions promising support was allowed to be blurred."
So Ukraine has no formal guarantees it can fall back upon, no clauses to demand action. There are, of course, vaguer provisions in the Helsinki Final Act of 1975 or indeed the United Nations Charter. But these do not carry any enforcement mechanisms. So, one can argue that there is an explicit moral commitment. But there is no binding legal commitment for U.S. intervention should Washington conclude that Ukraine has indeed, become a victim of aggression.
If the politicians who came to power in Kyiv after the flight of Viktor Yanukovych had known they were "on their own"--and that concrete, tangible support would not be forthcoming from Europe and the United States--that, in essence, they would have to come to some sort of modus vivendi with Moscow, might they not have been more amenable to the approach proposed by former National Security Advisor Zbigniew Brzezinski--to "practice policies toward Russia similar to those so effectively practiced by Finland" during the Cold War ("economic ties to both East and West, and no military association with any group perceived, by the Russians, to be anti-Russian")--which might not have been an optimal outcome, but certainly preferable to the risk of war we now face? Or were they given statements of support which were vague in formulation but could have reasonably been understood by them to promise much more robust support than they have received to date?
[This and all posts reflect only the personal opinions of the author]