FIVE MONTHS have passed since Secretary of State John F. Kerry declared that “the world must act quickly” to stop a “war of starvation” being waged by the Syrian regime of Bashar al-Assad against “huge proportions of the population.” It’s been nearly six weeks since the U.N. Security Council passed Resolution 2139, which ordered the regime and rebels to “promptly allow unhindered humanitarian access” and threatened “further steps” in the case of noncompliance.

Since then, according to U.N. humanitarian coordinator Valerie Amos, the war of starvation has worsened. The number of Syrians cut off from international aid has grown since January by 1 million, to 3.5 million. At least 180,000 people are in areas directly blockaded by government troops, which refuse to allow in supplies of food or medicine. In direct contravention of the U.N. resolution, the Assad regime has authorized aid convoys to cross only one of eight border posts identified by U.N. relief coordinators.

Ms. Amos reported to the Security Council on Friday that only 6 percent of the population living in besieged areas had received relief since the resolution passed. Meanwhile, she said, crimes against the population had escalated: Since Feb, 22, there had been 300 instances of sexual assault in and around Damascus. “The humanitarian situation,” she said, “remains bleak.”

The U.S. ambassador to the United Nations, Samantha Power, called Ms. Amos’s report “harrowing.” She said the Assad government “is the sole reason for the lack of progress in cross-border assistance” that “would allow the U.N. and its partners access to almost 4 million people.” She said: “The Assad regime’s murderous appetite for deploying artillery, ‘barrel bombs’ and airstrikes against civilians . . . is the No. 1 factor driving displacement and the broader humanitarian ­crisis.”

Naturally, reporters asked Ms. Power what she was proposing for the “next steps” cited in the resolution. That’s when the ambassador’s robust rhetoric suddenly went limp. “There’s nothing I can do and that we can do unilaterally to make the council do what we want,” she said. “So I can’t make any commitments.”

No, the United States can’t force action by the Security Council, where Russia, Assad’s ally, has a veto. But the Obama administration is not lacking in options to stop the ongoing, horrific crimes against humanity in Syria. What it lacks is the will to act. It could order the Assad regime to authorize border crossings by aid convoys — something Ms. Power said would require only “a stroke of the pen” — or face the same airstrikes Mr. Obama threatened last summer. It could target blockade points with drone or missile strikes. It could provide rebels with the air defense weapons they need to stop helicopters from dropping barrel bombs on civilian housing, hospitals and schools. It could disable the bases used by regime aircraft.

Ms. Power and her administration colleagues instead appear content to listen to “harrowing” reports from U.N. monitors, deliver angry statements and then throw up their hands because of their inability to win the cooperation of Vladi­mir Putin. It’s not a performance that will be judged well when historians consider why the world’s foremost power failed to stop this mass slaughter.