Northern Ireland hasn’t yet put the Troubles behind it. That’s the grim message from the recent detention of Gerry Adams, leader of Sinn Fein and one of the most prominent figures in Irish politics north and south of the border.

Unless care is taken, this terrible unfinished history could yet turn back to violence. The best way to avoid that dreadful possibility would be to draw a line under the crimes committed by all sides during the Troubles, and to cease prosecuting cases related to the conflict.

That decision should be combined with a process to lay bare the truth of who did what to whom.

Released after four days of questioning after his arrest on April 30 , Adams denied involvement in the Irish Republican Army’s abduction and murder of Jean McConville, a widowed mother of 10, in 1972. Police have sent the file to prosecutors.

Sinn Fein complains of a politically motivated arrest on the eve of elections to the European Parliament, and it questions the impartiality of the reformed Police Service of Northern Ireland in pursuing the investigation.

Both charges strike at the foundations of the settlement that has secured peace in Northern Ireland since the Good Friday Agreement of 1998. A prosecution of Adams, supposing the evidence calls for one, would stir fury in the nationalist community. Don’t dismiss the possibility that the still-unfinished peace process might unravel.

The danger has arisen because the agreement that secured peace made a lot of necessary but unpalatable compromises. Prisoners already convicted of conflict-related crimes received clemency, and qualified amnesty was offered in an ad hoc way to hundreds of others — but unresolved cases were left on the books and are still prosecutable.

In a conflict that killed more than 3,000 people and maimed far more, there are many such cases.

Here is the bleak truth: Justice demands that those crimes be prosecuted wherever possible, but peace requires the opposite. The closest Northern Ireland will get to reconciling those irreconcilable principles will be to combine a policy of no prosecutions with a tribunal to uncover the truth, along the lines of South Africa’s post-apartheid Truth and Reconciliation Commission.

Victims’ families on both sides of Northern Ireland’s sectarian divide want an accounting of what happened almost as much as they want to see the perpetrators punished; the truth alone would go some way to meeting their demand for justice.

A tacit decision not to prosecute cases, unsupported by any such uncovering of the history, might preserve the peace, but it can’t provide closure. And an undeclared policy of refraining from prosecutions, by failing to eliminate the risk of being sent to jail for past crimes, leaves in place the threat of violence and intimidation that even now oppresses both communities.

All sides — including members of the security forces — committed crimes during the Troubles, so an explicit no-prosecutions policy would hand no group victory over any other. That’s what makes it possible.

But with such a policy in place, would any role be left for a truth tribunal? Why would anybody apart from victims and their families testify?

The incentive of amnesty in exchange for truthful testimony would be undermined if a no-prosecutions policy were already in place — but truthful witnesses could be granted immunity in law, a stronger protection than a reversible decision not to prosecute.

Testimony in any event shouldn’t be compelled. Many might decline to testify, which would be revealing in itself — but, assured of immunity, many others would come forward, and new light would be shed.

Gerry Adams and Sinn Fein have previously said that an “independent international truth commission” is needed to advance a “genuine process of national reconciliation.” If such a panel were created, Adams might need to be reminded of his eagerness to take part, but that doesn’t make the proposal wrong. Reconciliation requires a coming to terms with the past. In Northern Ireland, that task has barely even begun.

Peace, truth and justice would be ideal. In leaving the Troubles behind for good, it will be necessary to settle for two out of three.

Clive Crook (ccrook5@bloomberg.net) is a Bloomberg View columnist.

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