YANGON, Myanmar — It is unfolding again: Troops have unleashed fire and rape and indiscriminate slaughter on a vulnerable minority, driving hundreds of thousands of civilians to flee and creating a humanitarian emergency that crosses borders.

A crisis in Myanmar that many saw coming has brought a host of uncomfortable questions along with it: Why did the world — which promised “never again” after Rwanda and Bosnia, then Sudan and Syria — seemingly do so little to forestall an ethnic cleansing campaign by Myanmar’s military? And what can be done now to address the urgent humanitarian calamity caused when more than half of Myanmar’s ethnic Rohingya Muslims fled the country over just a few weeks?

Outside Myanmar, criticism of its military has mounted. The United Nations secretary general, António Guterres, has urged “unfettered access” for international agencies and called the Rohingya crisis “the world’s fastest-developing refugee emergency and a humanitarian and human rights nightmare.”

President Emmanuel Macron of France has called it genocide. And there is talk, although still tentative, of the European Union’s renewing targeted sanctions on people culpable in the violence that has driven the Rohingya from Rakhine, a state in western Myanmar.