During the Cold War, the rules of defections were well understood. Those high-profile figures who changed sides could expect to be set up by their new hosts with a home, an income and immunity from prosecution for any past crimes. But those old rules no longer apply in the era of asymmetric warfare, humanitarian interventions and the International Criminal Court. The Foreign Secretary, William Hague, announced yesterday that there will be no immunity for Moussa Koussa, the Libyan Foreign Minister, who flew in to Farnborough airport on Wednesday night.
It is an understandable decision. Koussa has blood on his hands. He is deeply implicated in Libya's historic sponsorship of terrorism around the world and its assassination programme aimed at Libyan dissidents abroad in the 1980s. Justice demands that he should stand trial for any part he has played in those crimes.
But denying Koussa immunity does have problematic consequences. Mr Hague attempted to capitalise on the defection yesterday by noting that Muammar Gaddafi "must be asking himself who will be the next to abandon him". He also urged others close to the Libyan dictator to follow Koussa's example.
Yet the refusal to offer Koussa immunity makes it less likely that further defections will take place. If Gaddafi's henchmen fear that the consequence of defection will be prosecution, they will be more inclined to stick with the Gaddafi regime. Thus, denying immunity for regime-defectors could have the effect of shoring up support for Gaddafi at this critical phase of the conflict.
There is a similar dilemma over whether Gaddafi himself should be permitted to go into exile, an idea that has been floated in various Western capitals in recent days. Such a deal could bring a swift end to the conflict, but at the price of allowing a murderous tyrant to live out his time in relative comfort.
There seems to be a tragic trade-off between justice and diplomatic effectiveness here. Letting the likes of Koussa and Gaddafi evade prosecution for past crimes could save many lives and increase the chances of establishing a free Libya. But it would also be an affront to those who died in terror attacks ordered by these individuals.
Ultimately, justice should take precedence. This is not just about upholding the rights of victims; it is about the integrity of international law. If powerful nations are seen to be selective in its application, the deterrent effect of the existence of the ICC will be undermined. Brutal dictators will know that, should they one day find their backs to the wall, they will be able to push for a comfortable refuge abroad. The ICC will be perceived as a paper tiger.
There is a further consideration. Koussa was closely involved in rehabilitating Libya with the international community from the late 1990s. He played a key role in negotiating compensation for the families of the victims of the Lockerbie bombing and opening up Libya to Western oil interests. A trial could bring vital information about those murky deals to light. If Koussa were granted immunity, that information would probably remain buried for ever.
Yet amnesties and pardons for past crimes are not incompatible with justice, as our own experience with the IRA vividly demonstrates. The refusal to offer Koussa blanket immunity from prosecution is the right decision. But his defection and co-operation could be taken into consideration should he be placed on trial. It will not be easy, but a way needs to be found to reconcile international justice with international realpolitik.