Indeed, the government enjoys enormous public support among the majority ethnic Sinhalese for its May 2009 military victory against the rebels, the Liberation Tigers of Tamil Eelam. Mr. Rajapaksa is likely to use any international investigation to his advantage.

One reason the United Nations has found it difficult to bring about accountability, even long after the end of a gruesome war, is that the rebels were members of a widely despised, brutal insurgency that was deemed a terrorist organization by many countries in the aftermath of the attacks of Sept. 11. The Sri Lankan government deftly leveraged international support to crush the rebels.

The difficulty equally reflects political pressures that have nothing to do with Sri Lanka. India, for instance, has a large and influential ethnic Tamil population, and its leaders can hardly afford to alienate them ahead of India’s parliamentary elections, due to start next month. Nor can it ignore Sri Lanka’s ever-growing ties with China.

The war in Sri Lanka has roiled the United Nations. It had a large presence in the country during the last few months of the war, when up to 40,000 people are believed to have been killed, but was notably silent about the scale of killings. In an internal review, the United Nations described its silence as “a grave failure,” taking to task officials who “did not perceive the prevention of killing of civilians as their responsibility.”

That internal review led to a stark shift in policy, called “rights up front.” United Nations officials in the field are now under orders to report rights violations, and senior officials here are to engage in what Deputy Secretary General Jan Eliasson called “quiet diplomacy” or, if necessary, raise the violations with the Security Council.

The Security Council also did not pay close attention to rights abuses in a conflict that sought to stamp out the Tamil Tigers. “It was clear member states didn’t want to be told,” said Ian Martin, a former senior United Nations official.

He added that there was slim chance of punishing perpetrators now, without the government’s cooperation. “It’s one step further to a definitive account,” he said. “Truth is, there’s not going to be accountability.”