ON MONDAY, Vladi­mir Putin and Donald Trump spoke by phone about Syria and agreed on “the need to work together in the struggle against the No. 1 common enemy — international terrorism and extremism,” according to a Kremlin statement. Hours later, Russia and its Syrian allies launched a massive new bombing campaign against eastern Aleppo and other rebel-held territories. Just a coincidence? Not likely, given what we know about Mr. Putin.

There are no Islamic State forces in Aleppo, though Mr. Trump does not appear to be aware of that fact. There are an estimated 250,000 civilians who, according to the United Nations, received the last available food rations last week. There are also rebel forces that until now have been trained and supplied by the United States and its allies, as well as groups linked to al-Qaeda. Surrounded by Syrian, Iranian and Shiite militia forces since July, all face the same brutal ultimatum President Bashar al-Assad has delivered to other rebel-held areas: Surrender, or die through bombing or starvation.

Mr. Putin’s evident aim is to support the Assad regime in a campaign to overrun the city, and perhaps other rebel-held areas, during the 2½ months of the U.S. presidential transition. If so, the result will likely be the worst humanitarian catastrophe yet in a war that has already seen more than 400,000 people killed by bombing, chemical weapons, torture and other depravities. Yet neither the outgoing nor the incoming U.S. president appears willing to do anything to prevent this calamity.

President Obama was asked about Aleppo at his news conference Monday by a journalist who pointed out that the United States had intervened to prevent a similar assault on the Libyan city of Benghazi. “We don’t have that option easily available to us,” said Mr. Obama, who recently set aside several such options, such as grounding the Syrian air force. He added that the administration would continue to press for “humanitarian safe spaces and cease-fires” before conceding, “I recognize that that has not worked.” While the honesty was welcome, Mr. Obama’s apparent willingness to watch fecklessly as hundreds of thousands of people are starved and bombed during his final weeks in office is morally abject. It will deepen the ineradicable stain Syria will leave on his legacy.

Mr. Trump, for his part, has all but given Mr. Putin the green light for atrocities. While we don’t know the specifics of what was said in his conversation with the Russian ruler, the president-elect in an interview with the Wall Street Journal on Friday repeated that “Syria is fighting ISIS and you have to get rid of ISIS. Russia is now totally aligned with Syria.”

Again, the Syrian regime is not fighting the Islamic State in Aleppo. It is bombing and besieging its own citizens, with Russian and Iranian help. In refusing to allow aid deliveries and in targeting hospitals, it is willfully committing crimes against humanity. “I don’t think anybody wants a quarter of a million people to be starving in east Aleppo,” said Jan Egeland, the head of a U.N.-backed humanitarian task force. Tragically, he is wrong. The Assad regime and Mr. Putin want it. Mr. Obama is unwilling to prevent it. And Mr. Trump is, at best, indifferent.