IN LITTLE more than a month, a half-million Rohingya Muslims in Burma's Rakhine state have been forced into a terrified exodus to Bangladesh. They fled the Burmese military, which burned villages and reportedly shot men, women and children in retaliation for an attack on 30 police posts by a Rohingya militant group. The scorched-earth campaign against the Rohingya has been called a "textbook example of ethnic cleansing" by the U.N. high commissioner for human rights, Zeid Ra'ad al-Hussein. An urgent, stern and unmistakable response is necessary.

The de facto leader of Burma, Aung San Suu Kyi, has been justifiably criticized for failing to find her voice, the eloquence of a Nobel Peace Prize laureate, during this brutal crackdown. But the chief perpetrators are in the military, with whom she shares power. The army was undoubtedly furious over the militant attacks on police posts Aug. 25, but the subsequent response was entirely disproportionate, another chapter in the long and painful persecution of the Rohingya in Buddhist-majority Burma, also called Myanmar. Any response must now make clear to military leaders that inflicting such misery on a whole population is intolerable.

As a start, the Rohingya must be allowed to return, immediately. The worst thing that could happen would be if temporary flight turned into permanent expulsion. The government should allow more humanitarian organizations to bring aid to the Rohingya, and open up the region to international inspection and investigation, especially following the finding in late September from Human Rights Watch that the onslaught amounted to crimes against humanity under international law, including forced population transfer and deportation, murder, and sexual violence. The U.N. Security Council must move beyond mere discussion and forcefully express the world's disgust and support new sanctions.

President Barack Obama last year relaxed many of the U.S. economic and financial sanctions on Burma that had been imposed during the decades of military rule, a gesture to progress that included the 2015 elections that brought Aung San Suu Kyi's National League for Democracy to power. By lifting sanctions, Mr. Obama was not only acknowledging progress but also making a wager on future gains. The Rohingya calamity is a large reversal of those hopes. It is time for the Trump administration to impose targeted sanctions on military commanders and others who supported the Rohingya atrocities.

The military in Burma never entirely relinquished power. It still controls key ministries, a quarter of the seats in parliament and significant business enterprises. Twenty-two senators, led by John McCain (R-Ariz.) and Benjamin L. Cardin (D-Md.), have suggested to Secretary of State Rex Tillerson that the administration consider this a case for the Global Magnitsky Act, which allows the president to impose sanctions on individuals "responsible for extrajudicial killings, torture, or other gross violations of internationally recognized human rights." If the right people can be identified, this would seem to be what the law was made for.

More Rohingya are now in Bangladesh than in Burma. Failing to respond to their plight and persecution would compound the crime of their expulsion.