THE BATTLE for Aleppo is ending in catastrophe, both for the tens of thousands of people who have been besieged there and for the future of Syria. On Wednesday, Syrian government and Iranian-led Shiite militia forces renewed attacks on the last rebel-held streets of the city, shredding a promise to allow a peaceful evacuation. According to the United Nations, the pro-government forces have been executing civilians in the street or in their homes — including, on Monday, at least 11 women and 13 children. Thousands of men have been rounded up and gang-pressed into the Syrian army, or dispatched to an unknown but likely terrible fate. The United Nations’ term for this nightmare was apt: “A complete meltdown of humanity.”

The meltdown has several dimensions. One is the utter disrespect for the laws of war by the regime of Bashar al-Assad and its Russian and Iranian allies. These forces systematically destroyed hospitals, including pediatric facilities; decimated civilian housing with bunker-buster bombs and chlorine gas; and refused to allow food or humanitarian aid of any kind into the besieged districts of the city. Aleppo represents “the death of respect for international law and the rules of war,” David Miliband, the former British foreign secretary who now heads the International Rescue Committee, was quoted as saying . It sets a horrific precedent for conflicts in the 21st century.

The fall of Aleppo also means the elimination of any prospect in the foreseeable future for the end of Syria’s war or the waves of refugees and international terrorism it is generating. The Assad regime, which represents the minority Alawite sect, is unlikely ever to reestablish control over all of Syria, even with Russian and Iranian help. Even as it was crushing Aleppo, where Western-backed forces were based, it allowed the Islamic State to recapture the city of Palmyra. But the regime now will have no incentive to negotiate a peace settlement with the Sunni majority or Kurdish community. The likely result is years more of war and a steady stream of recruits for Sunni terrorist movements that target the West as well as Damascus.

Above all, Aleppo represents a meltdown of the West’s moral and political will — and in particular, a collapse of U.S. leadership. By refusing to intervene against the Assad regime’s atrocities, or even to enforce the “red line” he declared on the use of chemical weapons, President Obama created a vacuum that was filled by Vladimir Putin and Iran’s Revolutionary Guard. As recently as October, Mr. Obama set aside options drawn up by his advisers to save Aleppo. Instead, he supported the delusional diplomacy of Secretary of State John F. Kerry, whose endless appeals to Moscow for cease-fires yielded — as Mr. Putin no doubt intended — nothing more than a humiliating display of American weakness.

On Tuesday, Mr. Obama’s U.N. ambassador, Samantha Power, delivered an impassioned denunciation of the Aleppo carnage, which she said would “join the ranks of those events in world history that define modern evil, that stain our conscience decades later.” She excoriated the Assad regime, Russia and Iran but offered no acknowledgment that the stain of Aleppo extends also to her, the president and American honor. Those who will live with the long-term consequences of the Syrian catastrophe are unlikely to be so forgiving.