Ex post facto law is crucially important for libertarian theory. It is one of the bedrocks of our entire philosophy. This law declares, after the fact, that previous law was wrong, improper, anti-libertarian, and can punish people who obeyed it, even given that it was the law of the land. Without ex post facto law, the Nuremberg Trials would have been invalid. The Nazi defense, I was just following orders, I was just obeying the German law of the time, would be valid, without ex post facto law. Without ex post facto law, it would be logically impossible for there to be any justifiable reparations for slavery. For, before 1861, slavery was entirely legal in the US. How could slave-owning be considered a crime, when it was legal? Only libertarian law, based on the non aggression principle, coupled with ex post facto law, could declare slave holding as a crime, and this would be entirely justified.

The Best of Walter E. Block