The shooting down of Malaysia Airlines flight 17 has confronted Vladimir Putin with a dilemma he had sought to avoid: to continue to support the separatist insurgency in Ukraine in the face of a storm of international outrage, or cut the rebels off and allow them to be defeated by the government in Kiev.
Until the plane was hit by an anti-aircraft missile on Thursday, killing nearly 300 people, the Russian president had tried to hedge his bets according to circumstances on the battlefield and western pressure. He moved troops and tanks away from the border after the Ukrainian presidential elections in May, but moved them back in recent weeks.
Similarly, he initially appeared to distance himself from the rebels until Ukrainian forces under the newly elected president, Petro Poroshenko, made significant gains in the east, triggering a new supply of Russian equipment over the border, including anti-aircraft missiles.
The MH17 disaster forces his hand. Anything he does now will attract much more scrutiny. Arms shipments across the very porous Ukrainian border, which had until now been a threat to the Ukrainian armed forces, will henceforward be seen as a direct threat to the international community and a trigger for global outrage. But pulling the plug on the separatists would leave them vulnerable to Ukrainian forces, which can be expected to seize the opportunity to crush the revolt, handing a strategic defeat to Putin.
It is already clear from Friday's UN security council meeting that if the rebels are found to have carried out the outrage with a Russian weapon, Moscow will find itself more isolated than at any time in its recent history.