The Russian missile cruiser Varyag on patrol in the eastern Mediterranean. Russian warships equipped with long-range missiles cruise off Syria's coast to back the air campaign in Syria. (Vadim Savitsky/Russian Defense Ministry Press Service via Associated Press)

AN EXTRAORDINARY new crisis is beginning to unfold in Syria, a country that already has suffered through some of the worst war crimes, humanitarian depredations and refu­gee flows in recent history. Russia, Iran and the Syrian government are conducting a major offensive aimed at recapturing the city of Aleppo and the rebel-held territory that connects it to the border with Turkey. They have cut one supply route to the city and are close to severing another, trapping rebel forces along with hundreds of thousands of civilians. Tens of thousands have fled to the border, where Turkey is denying them entry.

This campaign is being waged in open defiance of a U.N. Security Council resolution adopted in December, which required the Syrian government to provide humanitarian access to areas under siege and demanded an end to the shelling and bombing of civilian areas. Russia, which voted for the resolution, is indiscriminately bombing civilian targets in the Aleppo area, using banned cluster munitions, according to Human Rights Watch. Iranian commanders are on the ground, directing attacks by Shiite fighters from Lebanon, Iraq and Afghanistan.

In the face of this onslaught, which promises to destroy any chance of an acceptable end to the Syrian civil war, the Obama administration has been a study in passivity and moral confusion. President Obama is silent. Secretary of State John F. Kerry has been reduced to reading the text of Resolution 2254 aloud, as if that would somehow compel a change in Russian behavior. Mr. Kerry on Tuesday faulted Moscow for “making it much more difficult to be able to come to the table” for the peace talks he has been trying to broker, which were stillborn last week in Geneva.

Yet Mr. Kerry also has been blaming the Syrian opposition, which succumbed to U.S. pressure to turn up for the talks but refused to go forward while the bombing continued. Russia and Iran, Mr. Kerry insisted Friday, were ready to support a cease-fire; that it hadn’t happened, he suggested, was because the rebels had not “come to the table.”

That conclusion is as preposterous as it is self-serving. Russia launched bombing operations in northern Syria within 48 hours of the Security Council vote on Dec. 18 and never stopped; it has long been obvious to almost everyone that the regime of Vladi­mir Putin is seeking a military victory over Western-backed rebels, not a truce. Mr. Kerry conceded last week that Moscow might have lulled him with “talk for the sake of talk in order to continue the bombing,” adding, “We will know that in the course of the next days.”

If he is honest, he will now acknowledge his error. By insisting on the Geneva talks — and reducing support to the rebels as a way of forcing their participation — the United States has paved the way for the ongoing military debacle. Having predicted that Russia would find itself in a “quagmire,” Mr. Obama now is at risk of watching as Mr. Putin eliminates any non-terrorist alternative to the Assad regime, and thus strengthens the Islamic State.

It might still be possible to rescue the Syrian opposition and the hundreds of thousands of civilians at risk — but only if the United States and its allies act quickly to bolster rebel forces and create havens for refugees. The alternative — to hope that Russia and Iran stumble, or suddenly embrace a truce — has already been proved a fantasy.