BANGKOK, Thailand — Leaders in Sri Lanka can claim what much wealthier and better-armed nations cannot: the country’s terrorism movement has been thoroughly vanquished.

Situated off India’s east coast, Sri Lankan authorities spent more than three decades battling the Liberation Tigers of Tamil Eelam. Better known as the “Tamil Tigers,” the guerrilla separatists were finally defeated last year in an all-out army assault.

One year later, Sri Lankan leaders are trumpeting the dawn of a vibrant, terrorism-free society.

“There is a unique quality about this moment,” said Sri Lankan Foreign Minister G.L. Peiris, in a rare meeting with foreign journalists in Bangkok this week. “People sing the national anthem today with much more feeling and vigor.”

Chinese and Indian investors are building ports and railways, he said. Waters once thick with sea-faring Tamil Tiger guerrillas are now open to fisheries. In the most populated city, Colombo, investors will build hotels to support a coming tourism bonanza.

“Whatever the intrinsic strengths of the country were, we were not able to derive the fullest benefit because of escalating violence,” Peiris said. “It is that which we have put behind us.”

But as Sri Lanka forges ahead, a growing chorus of human rights groups — and the United Nations — insists its leaders must atone for alleged war crimes.

“These people will be held accountable for war crimes,” said Brad Adams, executive director of Human Rights Watch’s Asia division. “I’m not saying imminently. But the evidence is so strong, and they’re so unpopular internationally, that a time will come when no one will be there to protect them.”

As the Sri Lankan government often notes, the U.S. Federal Bureau of Investigation considered the pre-defeat Tamil Tigers “among the most dangerous and deadly extremists in the world.”

Their civil war, waged to create a sovereign nation for the Tamil ethnic minority, began in 1976. It has left more than 70,000 dead on both sides since the mid-1980s, according to the FBI.

An FBI fact sheet on the Tigers says the group “invented the suicide belt,” inspired Al Qaeda and placed “operatives right here in our backyard, discreetly raising money to fund its bloody terrorist campaign overseas.”

The Tigers even maintained a ragtag air force, a navy fleet and a ground force armed with AK-47s and rocket launchers. So despised are the Tigers that electro-pop musician M.I.A., the daughter of a Tamil separatist, has received intense criticism for lyrics that only vaguely criticize the Sri Lanka’s attack on the rebels.

But rights groups claim even the ruthless Tigers weren’t deserving of the brutality Sri Lanka’s military unleashed in May 2009.

In routing the rebels, the army established a “no-fire zone” along the coastline that attracted civilians fleeing to safety. Then they shelled it, Adams said. Video evidence and testimony collected from witnesses and amateur footage suggest a civilian death toll of more than 20,000, according to Human Rights Watch.

Perhaps even more incendiary is a lengthy report issued by the International Crisis Group, which claims the army intentionally shelled medical stations, food distribution points and hospitals. Equally heartless were the Tigers, whom the report accuses of forcing civilians to fight and shooting those attempting to escape the war zone.

Watchdogs place special emphasis on execution footage, believed to be captured on a Sri Lankan soldier’s mobile phone. The clip depicts troops standing over barely clothed men and women lying bound and blindfolded in the mud. Instead of taking the presumed Tiger combatants as prisoners, the soldiers in uniform appear to fire bullets into their heads.

“The government of Sri Lanka did a detailed technical analysis of this video,” Peiris said. “It’s a fake.”

But a team of outside experts commissioned by the United Nations “strongly suggest” the footage is authentic. Special Rapporteur Philip Alston, an expert on extrajudicial executions, dissected the video with specialists in forensics, video composition and firearms.

The panel, he wrote in his technical analysis, “found the body reaction, movement and blood evidence was entirely consistent with what would be expected in such shootings.”

Human Rights Watch also claims to have identified one of the dead, a female Tamil Tiger lieutenant colonel. The army itself acknowledged it killed the same woman in an online account of Tamil Tigers killed during the “Last Battle.”

This footage, rights groups contend, is the centerpiece in a body of evidence that demands a U.N. war crimes investigation.

“When this government came to power they loved us. We’d been trashing the [Tamil Tigers] and they would cite us as an authoritative source of information,” Adams said. “I warned them. ‘You guys have the moral high ground. Keep it.’”

Instead, he said, leaders have soiled their credibility by dismissing solid fact-finding as “a vast plot” to disseminate faked evidence.

The government’s narrative of the final beach battle, however, could not sound more dissonant.

What outsiders call a massacre was in fact a “humanitarian mission” to liberate unwilling human shields from Tiger territory, said Defense Secretary Gotabhaya Rajapaksa to a government-appointed reconciliation commission in August.

The army racked up zero civilian casualties, said Rajapaksa, alleging the civilian body count tallied by outsiders included many plainclothes Tiger combatants. “Soldiers had to move in, identify and shoot,” he said. “That was a step we took to prevent civilian casualties. Nowhere in the world is this done.”

The Sri Lankan military’s alleged tactics — taking no prisoners, refusing to distinguish between guerrillas and civilians — has become known as the “Sri Lankan Model” in defense journals. Other nations with their own ethnic insurgencies from Burma to Israel have reportedly taken note of its success. A Jerusalem Post op-ed published last year urges generals to consider its potential effectiveness against Palestinian terrorists.

In 2010, the first year in decades free of the Tigers’ terrorism campaign, the Sri Lankan economy is expected to grow by more than 8 percent.

Times ahead, Peiris said, will harken back to the Sri Lanka’s brightest era: the prosperous years following the nation’s independence from colonial Britain. Then, he said, per-capita wealth exceeded that of Thailand and Korea.

He speaks glowingly of fertile waters “not fished for two-and-a-half decades because of violence” and parts of the island that claim “more species of mango than other countries have varieties of fruit.”

Investments in solar and wind power are near, Peiris said. “The economy is forging ahead,” he said. “We have avoided ... the potential disintegration of the social fabric.”

Watchdogs such as Adams see a much darker future for Sri Lanka’s leadership. “The faster they come up, the harder they’ll fall,” he said. “They think they’re building a dynasty. But they won’t succeed.”

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