The Americans, Churchill supposedly said, could be relied upon to do the right thing, but only after they had exhausted all the alternatives. With Vladimir Putin, you might be tempted to turn that aphorism upside down: after toying with all the better options, he turns around and plumps for the absolute worst.
After appearing to embark on a more co-operative foreign policy – helping the US with transit arrangements for its Afghan withdrawal; facilitating a deal to remove chemical weapons from Syria and entertaining the world at the Sochi Winter Olympics – Putin now seems to be on the verge of plunging the region if not into World War Three, then into a 21st century re-run of the Crimean War.
Is he really prepared to sacrifice the diplomatic gains of the past year to keep Ukraine inside Russia's sphere of influence in what looks very much like a fit of personal pique?
From the Kremlin, the picture, and the options, will look very different.
A common view from abroad is that Putin runs Russia single-handed, as a latter-day tsar, and indulges his own caprices. As a footnote, it has been suggested that the success – or, at least, non-failure – of Sochi might have emboldened him to show (even) more assertiveness in the neighbourhood.
Such views disregard both the continuing weakness of central power in Russia and the extent to which any Russian leader now must take account of public opinion.
Within Russia, the popular pressure on Putin will not be for restraint, but for action. His stance reflects a domestic consensus that, while Ukraine may be independent, its natural place is within Russia's orbit and Moscow cannot just stand by while the West conspires to snatch it away. "Who lost Ukraine?" is a question that has real potential to erode Putin's power.
Which is why, more remarkable than Putin's current threat to use force, was the relative calm with which he initially responded to the Kiev protests and the collapse of the Ukrainian administration. He even sent an envoy to join the EU foreign ministers brokering a deal between Yanukovych and the opposition and looked ready to accept the outcome.
It was when that deal collapsed, violence broke out and the conflict spread that Putin started to play hardball.
And he did so in a way, it is worth noting, that carefully mirrored certain Western practice: a limited and deniable show of force (in Crimea); a vote in Parliament, and reference to a "responsibility to protect" an endangered population.
It is too simple to cite age-old cultural and religious ties as a reason for war over Ukraine, though they count for much, and Putin has a fierce sense of Russia's dignity.
Nor is it because of fears that, one day, Ukraine might join the European Union. (Though this might be because Putin knows better than Kiev how tough the entry conditions are).
I have heard him say categorically that he accepts Ukraine as a sovereign independent state and, equally categorically, that it is up to post-Soviet states to determine their own economic course if – and it is a big if – they can afford the cost of loosening economic ties with Russia.
The appeal of the EU is not itself a problem.
There are, though, two other reasons that may dictate Putin's current actions.
The first derives from the 1990s and concerns the West's betrayal – as Russia sees it – of its pledge not to expand Nato to Russia's borders.
One view is that this promise, given to then President Gorbachev, lost its validity when he lost power. This is not the view subsequent Russian leaders have taken.
Putin, and even his usually mild-mannered prime minister and defence minister, become utterly incensed when they talk about it.
They see it as a prelude to humiliation and "encirclement".
The all-or-nothing, now-or-never, East-or-West language used by EU leaders, when they tried to persuade Ukraine to sign up to the EU association agreement before the Brussels-imposed November deadline, only strengthened Russia's suspicion.
Those Western officials – John Kerry and William Hague among them – now saying that any solution for Ukraine must recognise its position between East and West and involve Russia were singing a very different tune late last year.
The other reason for Putin's threat is the gulf that has opened up between the way in which recent events in Ukraine are seen in Russia (and in eastern Ukraine and Crimea) and the way they are seen in the West.
Putin might still sacrifice Yanukovych – a leader who cannot keep order is no good as an ally, anyway – but the fact that he was democratically elected, the way in which he was toppled (by street protests) and the far-Right elements in the crowd (presented in Russia as a majority) all combine to foreshadow a new regime that could, in Moscow's view, indeed imperil Ukraine's Russian-speakers.
This may not be the truth of the situation, but it is one that many Russians, including Putin, who were educated in Soviet times, find credible – more credible, at least, than the Western version of enlightened protesters removing a despot in the national interest.
So long as these two versions of recent events persist, so, too, will the prospect of armed conflict.