The war against terror has forced governments to rethink national security. Protecting against invisible, anonymous threats requires extraordinary vigilance and exceptional measures. Ultimately, victory in this battle will rest on a broad consensus on what we are fighting for; only then can governments marshal their resources against extremists. Yet there is a very real danger that measures taken to fight terrorism will undermine the will needed to prevail in that struggle. We cannot afford to become like our enemies. Two recent events should remind us of how easy that is, and of how hard we must work to maintain the moral clarity that separates us from the terrorists.

In one case, Indonesia's Constitutional Court ruled recently that antiterror laws passed in the wake of the Bali bombings could not be used retroactively. In a 5-4 decision, a majority held that the government could not use tough new laws to punish behavior that occurred before the laws were passed. That is common sense. Ex post facto laws can easily become capricious and instruments of oppression.

The court decision raises questions about the fate of more than 30 terrorists, including those found guilty of killing more than 200 people in the October 2002 nightclub bombings in Bali. The ruling does not mean that the terrorists will go free, as the court pointedly declined to throw out the convictions. Many of the guilty men confessed, and there is considerable physical evidence to tie them to the crimes. Most are likely to be retried under older statutes against murder, conspiracy, or aiding and abetting fugitives. The lower level of proof allowed by the new laws should have an effect on judicial reasoning. There is also some question about the risk of double-jeopardy: No person can be tried twice for the same crime. The first conviction, though, may preclude the need for a second trial.