Peter Vickery explains: The Latin expression ex post facto literally means: ‘after the fact’. In the law the expression is often used to describe an ex post facto law, or what is commonly called a ‘retrospective law’.

An ex post facto or retrospective law has its most serious consequences in the criminal law. The effect of such a law is to convert conduct which is innocent at the time it was undertaken to punishable criminal conduct by the passing of a law at a later time.

A simple example will serve to illustrate the point: Kate regularly drives her daughter to school following her usual route. She travels through a 60 kph speed zone near the school at 50 kph. Her speed is recorded on a traffic camera. Kate’s driving has not broken any law and she is perfectly innocent. Parliament then passes a law converting this particular speed zone to a 40 kph zone. This is a legitimate and sensible exercise of legislative power. However, the new law goes a step further: it says that the new speed zone shall also apply at an earlier date—which includes the time that Kate was driving her daughter to school. Kate’s earlier innocent conduct overnight becomes illegal. She is justifiably outraged when she receives hefty a fine and feels a profound sense of injustice and insecurity: ‘What other conduct of mine will they punish me for?’ she rightly asks.

What then has all this to do with David Hicks, imprisoned in Guantanamo Bay, Cuba? In the events that have happened, as we shall see, these unassuming little words have taken on central importance to his case.

Following his capture in November 2001, David Hicks was not charged with any offence until 26 August 2004. He was then charged before the former military commission with three offences: Conspiracy, Attempted Murder by an Unprivileged Belligerent and Aiding the Enemy. Since the Hamdan decision was handed down by the US Supreme Court in June last year and after the recent ruling of the Convening Authority, all three of these offences have been abandoned. Hicks therefore remained imprisoned in Guantanamo Bay for more than five years without being served with a valid charge.

Earlier this month, he was formally charged with the novel offence of Providing Material Support for Terrorism. This is a recently invented war crime which was created on 17 October 2006 when President Bush signed the Military Commissions Act of 2006 into law in the United States—yet it is sought to apply the offence to the activities of Hicks in Afghanistan in 2001.

Keep reading (ABC RN’s Lingua Franca transcript).