OVER THE past two decades governments in several unstable parts of the world have tried to end violent conflicts or consolidate fragile democracies by forgoing the full prosecution of dictators and war criminals in exchange for the peaceful acceptance of a new political order. For the most part, the policies have proved remarkably successful. Chile and Argentina used them to implant democracy after decades of dictatorship, while South Africa made a peaceful transition from apartheid to majority rule. In time, even the sacrifice of justice has been mitigated: As their democracies have grown strong, the South Americans have repealed the amnesties they once granted to military rulers and reopened some of the most important human rights cases.

Colombia, a large South American democracy bled for generations by rural insurgencies, now seeks to employ this tool. Its Congress recently voted to grant limited immunity to thousands of right-wing fighters known as "paramilitaries." In exchange for turning in weapons and disclosing information about their organization and financial assets, the militants would be eligible to receive quick trials and prison terms limited to a maximum of eight years. Those involved in drug trafficking, as many of the paramilitary groups have been, would be exempted from extradition to the United States.

The government of President Alvaro Uribe, one of the region's closest U.S. allies, frankly describes the deal as a painful compromise that nevertheless offers the prospect of demobilizing up to 20,000 of the paramilitaries. It seeks millions in aid from the United States and other rich donors to fund resettlement programs. It has the Bush administration's support but has drawn objections in Congress, both from Democrats and Republicans.

Part of the resistance comes from representatives disturbed by the idea that the paramilitaries would be held less than fully accountable for crimes that include massacres and acts of terrorism in addition to cocaine trafficking. These objections overlook the fact that Colombia's program stands to deliver considerably more in penalties than those previously tried in Latin America, which were mostly based on full amnesties. Why, the Colombians reasonably ask, should foreigners overrule their judgment, reached by a democratic vote in Colombia's Congress, that the trade-off of justice for peace is worth making?

A more serious argument is that the Colombian plan won't work. The penalties for militants who lie about their crimes or assets, critics contend, are virtually nonexistent, which means leaders may escape with most of their assets and organizations intact. The risk is that after a few years spent in relatively comfortable jails, the paramilitary leaders will return to the business of trafficking drugs, terrorizing rural communities and penetrating the Colombian government.

U.S. critics contend Mr. Uribe's government can force the paramilitaries to accept tougher terms. In reality what they are calling for is a return to war between the government and the insurgents -- a war that Mr. Uribe has waged more vigorously and effectively than previous Colombian presidents but that remains unwinnable. If the demobilization plan fails, Colombia will have no choice but to return to fighting -- and paramilitary leaders who continue to traffic drugs will once again face extradition to the United States. In the meantime, the United States ought to do what it can to give this crucial initiative by a democratic ally every chance to succeed.