Human rights campaigners in New York yesterday promised that by the end of the year criminal charges would be filed against one of the men it accuses of murder, torture and other atrocities in a list submitted to the United Nations Human Rights Commission.
The international campaign group Human Rights Watch (HRW) refused to name their first target, but said that a large and deserving field had already been narrowed to three. All that Reed Brody, advocacy director, would reveal was that Idi Amin, who lives in safety in Saudi Arabia, housing and pension provided by the government, is not the first target.
"But a criminal case will be filed in a country where the accused is living, with the help of organisations from the country he fled," he said.
The prosecution of the former dictator will be a test of the claimed swing towards human rights since the unprecedented legal action against General Augusto Pinochet of Chile.
Fired by the landmark case, campaigners are keen to retain momentum. Post-Pinochet, victims of dictatorships across the globe have called HRW's New York office to ask them to seek out previously unexplored laws now that the idea that former heads of state - and in the case of President Slobodan Milosevic, sitting heads of state - are no longer immune from prosecution.
"We described the Pinochet case as a wake-up call for tyrants round the world but it was also a wake-up call for victims," said Mr Brody. "The laws that were involved in the prosecution of Pinochet have been round for a while. They had just not been used."
The case against the former dictator of Chile came to depend on an international torture convention which the UK signed up to in 1988. In the past few months HRW has reviewed the countries dictators are living in and examined international treaties - from the torture convention (to which 114 countries are signatories) to the Geneva Conventions and laws governing crimes against humanity - that might be used.
Amin has been ruled out for now because of Saudi Arabia's poor human rights record. It would be unlikely to co-operate with legal action, and Western countries would not apply pressure on a country rich in oil and generous with arms contracts.
Mengistu, who lives in Harare under the protection of his old friend and ally, the Zimbabwean President Robert Mugabe, is also on the list. He is accused of killing 200,000 people during his 17-year rule in Ethiopia. Millions more died in famine and civil war during that time. He is safe from legal action while Mr Mugabe remains in charge but the President's position is increasingly precarious.
Also listed are Duvalier, who made his home on the French Riviera after fleeing Haiti in 1986. Accused of thousands of political murders, Duvalier is reported to be flitting from apartment to apartment among Paris's exiled Haitian community.
HRW also names Hissein Habre, former dictator of Chad, now in Senegal; Emmanuel Constant, former right-wing paramilitary chief in Haiti, now in New York; Alfredo Stroessner, former leader of Paraguay, now in Brazil; and Raul Cedras, former army chief of Haiti, currently in Panama.
But activists face many obstacles. "The Pinochet case is a wonderful precedent but also very sobering," says Mr Brody. "It was really 30 years in the making. It shows how much is needed to mount a prosecution. It is very difficult to establish a chain of command for atrocities.
"Then you need evidence that will stand up in court and then an aggressive prosecutor willing to try to prove a case in a country where the atrocities did not take place and which might think it has more pressing priorities."
The International Criminal Court, planned by the UN, will, human rights groups believe, be enormously helpful in the prosecutions of dictators. But it will be at least five years before it is operating.
And there are countries opposed to its creation, appalled at the idea of outside interference their in internal affairs. Which is why the US found itself in such dubious company when it and six other UN members - including China and Iraq - voted against the court.
Nevertheless a new mood does seem to have emerged. "It was the political will that was lacking before," said Mr Brody. "Britain did the right thing in executing the warrant issued by Spain against Pinochet."
At the end of the year another government may have the chance to prove its mettle.