The Tamil Tigers’ defeat presents the democratic world with a quandary. Does it punish Sri Lanka for its brutal conduct of the war and risk hampering reconstruction efforts and pushing the island deeper into the embrace of China, Iran and other rival powers?

Or should it overlook multiple alleged abuses, and risk setting a precedent for future conflicts, and dashing hopes for reconciliation between the Sinhalese and Tamil communities?

The answer could either consolidate or undermine a decade of relative progress in enforcing international laws of war, most notably in the former Yugoslavia, Liberia and Sierra Leone.

“Sri Lanka is a test for the international community in terms of holding people to account for atrocities,” said Sam Zarifi, Asia Pacific director of Amnesty International.

Britain, the EU and the UN rights chief are calling for an investigation into whether Sri Lanka committed war crimes by firing on civilian targets, including hospitals. They won an early victory yesterday by persuading the necessary one third of the 47 members of the UN Human Rights Council to convene a special session next week.

But the Council has no real power and is still dominated by China, Russia and Middle Eastern and African nations that routinely block scrutiny of each others’ human rights records.

Even if it recommends setting up an international commission of inquiry, that would rely on co-operation from Sri Lanka’s government, which denies there are any grounds for a probe.

Similar commissions in Bosnia and Darfur led to war crimes prosecutions, but in both those cases, investigators had better access on the ground than they will likely get in Sri Lanka.

The only other international body that can investigate war crimes is the International Criminal Court, which does not have jurisdiction over Sri Lanka as is not a signatory state.

The ICC, based in The Hague, can take up a case in a non-signatory state – as it did with Darfur – but only if it is referred by the UN Security Council, where Sri Lanka has support from China and Russia.

One alternative is to block Sri Lanka’s application for a $1.9 billion emergency loan from the International Monetary Fund. The United States – which has the most voting rights on the IMF’s executive board – has already said it is not ready to support the loan, as has Britain.

But many officials in Washington and London are reluctant to attach political conditions to IMF loans, while others say withholding funds now would only harm civilians. The same logic applies to the EU suspending its GSP Plus scheme which waives import tariffs for Sri Lankan garments.

Economic sanctions might also encourage Sri Lanka to seek more aid and investment from China, Iran and Libya – and grow even more dismissive of Western human rights concerns.

That leaves only the unilateral approach, under which Western countries could launch their own investigations – as the US did in Darfur – and take action through national courts against individual Sri Lankan officials.

In theory, any US or European court could take up such cases, especially if they involve victims who have dual nationality – as many Tamils do.

Washington has an intriguing additional option in that Gotabaya Rajapaksa, Sri Lanka’s Defence Secretary, is a US citizen. That means he could, in theory, be prosecuted under the War Crimes Act – just as Chucky Taylor, the son of the former Liberian President, was prosecuted in the US for torture committed in Liberia.

If such charges were brought against Mr Rajapaksa in the US, he could be detained and extradited while travelling overseas.

“This is an age in which mechanisms for holding leaders accountable are expanding and developing,” said Tom Malinowksi, Washington advocacy director for Human Rights Watch.

“There’s still no reliable international system, but these leaders do face some jeopardy. They’re not off the hook yet.”

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