Western leaders who'll be meeting for emergency talks in Europe next week have an unusually important judgement call to make: do they believe president Putin when he says he intends to go no further in Ukraine?
It may well be the most important decision facing Western policy-makers since they gave the green light to the reunification of Germany in 1990. Europe's future hangs in the balance.
If they decide that president Putin is likely to be satisfied with having reabsorbed Crimea back into the bosom of mother Russia, well, that's something the West can live with. Crimea is not a strategic Western interest, even if the principle of territorial integrity (one state does not gobble up bits of another state) has been flagrantly breached.
If, on the other hand, they suspect that the Russian president does intend to bite off another bit of Ukraine - the eastern part where most people are Russian-speakers and where many feel a closer affinity to Moscow than to Brussels - that will be a step too far.
And it's at that point that we would enter uncharted waters. No one envisages going to war with Russia, although it's worth reminding ourselves that if Mr Putin were to move against any of the Baltic states (Lithuania, Latvia, Estonia), all of which are members of both the EU and Nato, then there'd be no alternative to war. An attack on one Nato member state is regarded as an attack on them all, and a military response would be all but inevitable.
That's how serious this is. So far, there has been no sign that Russia does intend to up the ante, although the annexation of Crimea and Moscow's justification that it was defending its Russian compatriots does create a dangerous precedent.
(By the way, if you can't find the Moldovan region of Trans-Dniestr on a map, now may be the time to look for it. Some analysts are already highlighting it as a potential next flash-point.)
As for the Baltic states, what would Mr Putin do if ethnic Russians in Riga or Vilnius were suddenly to "ask" for protection? Could he blithely ignore them, having gone to the aid of ethnic Russians in Crimea? Has he perhaps embarked on a course without fully having considered where it might take him?
It's easy to assume that he's having things all his own way and that the annexation of Crimea was part of a carefully calibrated strategy. Maybe it was, but it may also have been a tactical response to a crisis in Kiev that from Moscow looked like being seriously damaging to Russian interests.
Imagine what the world looks like as you stare out of a Kremlin window. The US is worn down by wars of attrition in Afghanistan and Iraq - and the EU has barely emerged from a financial and economic crisis that threatened to tear it apart. This, surely, is the moment to reassert your right to defend your own backyard: no more Nato encroachment up to your borders, no more EU blandishments to tempt your neighbours.
Since 1989, Moscow has watched helplessly as the Baltic states, plus Poland, Hungary, Romania, Bulgaria, the former east Germany, and the former Czechoslovakia have all moved out of the Soviet/Russian orbit and into the EU/Nato camp. It has been, in the eyes of Mr Putin, a massive humiliation -- and the West's forays into military adventurism (Afghanistan, Iraq, Kosovo, Libya) have simply rubbed salt in the wounds.
Now, president Putin has decided that enough is enough. He's under growing political pressure at home as the Russian economy splutters, so what better time to wrap the Kremlin in the nationalist flag and unite Russian voters in support of their compatriots in neighbouring states? In the words of the former British ambassador Sir Christopher Meyer: "Putin and his cabal of close advisers are moved by a poisonous combination of grievance and ultra-nationalism."
That's why, he says, "there is no possibility that any combination of economic sanctions and visa restrictions currently under consideration in the West will check the Kremlin. Crimea is gone for good."
On the other hand, Russia's economic weakness may turn out to be the West's strongest card. That, at least, is the thinking that underlies the current taste for imposing sanctions. It was, after all, economic weakness that played a significant role in the ending of the Cold War and the collapse of the Soviet empire - and it may turn out to be an equally significant factor again. Two credit rating agencies have already downgraded Russia post-Crimea, and that does mean a real added cost to Russian borrowing on international markets.
The response so far from Western leaders to president Putin's Crimea-grab has been, in effect, to shout loudly while wielding a stick so small as to be almost invisible. They must do better next week. By all means keeping turning the sanctions screw, but more importantly, lay out unambiguously the nature of the Nato mutual defence commitment as it applies to the Baltic states.
Mr Putin may well be thinking that just as the US could get away with doing pretty much whatever it liked in the first 20 years after the collapse of the Soviet Union, now it's Russia's turn again. He must be disabused of that notion as quickly as possible - for our sakes as well as for Ukraine's. A world in which one superpower can operate overseas unchecked is a highly dangerous one. Surely that's one lesson we should have learned by now.