Luis Posada is a 77-year-old Cuban exile who has been involved in many attempts to overthrow Fidel Castro since the abortive Bay of Pigs operation in 1961. He has long been seen as a prime suspect in the 1976 midair bombing of a Cuban airliner that killed 73 people. He was arrested in Caracas many years ago and charged with the offence but escaped from custody in suspicious circumstances. He has since made his way to Florida, a place that has, over the years, become something of a rest-home for the heavy-mob enforcers of Latin American military dictatorships.

The Posada affair is top of the agenda in Cuba, where Fidel Castro has this week been repeatedly calling on President Bush to act decisively against terrorism by arresting Mr Posada and deporting him for trial. The case is an important one because at its heart is the belief, held in many parts of the world, that the US has one standard of morality for its allies and another for its enemies.

By a twist of fate, the Posada case has come to prominence just as the US state department refused admission to Dora Maria Tellez to take up a visiting professorship at Harvard. Ms Tellez is as well known in Latin American political circles as Mr Posada. As a young Sandinista, she played a major role in the overthrow of the dictator Anastasio Somoza in Nicaragua in 1979. One activity she was involved in was the armed occupation of the national palace, an event which sent a signal to Nicaraguans that Somoza was vulnerable and that the revolution against him could succeed.

Ms Tellez has visited the US many times since the revolution, both as an academic and a visitor. No one in the state department seriously imagines that she is a terrorist threat any more than they believe that Nelson Mandela constitutes a risk because of his involvement with the African National Congress, which was once described as a terrorist group. The refusal to let Ms Tellez pursue her academic career has more to do with spite and the fact that the American diplomats who played such a shabby role in central America in the 70s and 80s are now back in favour. The decision to bar her from entry, however, acts as an educational counterpoint to what has been happening with Luis Posada.

Some American former diplomats have had the courage to speak up about Mr Posada. Wayne Smith, once his country's chief diplomat in Cuba, said this week that "the only acceptable action here is to expel Mr Posada from the United States. We must not harbour terrorists." The state department response has come from Roger Noriega, who has responsibility for western hemisphere affairs and who said that the charges against Mr Posada "may be a completely manufactured issue". Only a few days ago, Mr Noriega said with a straight face that he did not even know if Mr Posada was in the US. One hopes he was made aware of yesterday's arrest.

Mr Posada spent many years on the payroll of the CIA, who were happy to turn a blind eye to his activities and those of his associates as long as they were pointed in the direction of Fidel Castro. He must have imagined that he would spend the evening of his days sipping mojitos in Miami. Following his detention, the state department has to make its choice: it can acknowledge that blowing up civilian airliners is unacceptable even when the victims are citizens of an "axis of evil" country and duly dispatch Mr Posada for trial to Venezuela, or it can ignore the issue and face further international cynicism.