President Obama, right, with Colombian President Juan Manuel Santos at the White House on February 4. (Carolyn Kaster/Associated Press)

COLOMBIAN PRESIDENT Juan Manuel Santos basked in bipartisan acclaim when he visited Washington this month, and no wonder: At a time when Americans are doubting the nation's ability to manage foreign military interventions, Mr. Santos was here to celebrate one that succeeded. Plan Colombia, a 15-year effort to support the Colombian army against rural-based militants deeply enmeshed in drug trafficking, has brought about what looks like the end of what Mr. Santos calls "the oldest, cruelest armed conflict in the whole of the Western hemisphere." It is, as The Post's Karen DeYoung reported, "widely considered one of the most successful U.S. assistance efforts in history."

That wasn't always the case. When Plan Colombia was first proposed by President Bill Clinton in 2000, human rights groups and congressional Democrats carped that it focused too heavily on aid to the Colombian army, which appeared to be losing the fight against both the Revolutionary Armed Forces of Colombia (FARC) and right-wing paramilitary groups. As the New York Times reported at the time, "there was concern that a program of such size could inexorably lead to a broader American military commitment to the war and the potential for American soldiers to be drawn into combat."

That didn’t happen. Instead, under former president Álvaro Uribe and Mr. Santos, who served as his defense minister, Colombia beefed up and professionalized its army with U.S. help, allowing it to gain a decisive military advantage over the FARC and right-wing paramilitary groups. The result was successful peace talks, first with the paramilitaries and now with the FARC, which is expected to complete a deal with the Santos government to disarm and sever its links with drug trafficking by late March or early April.

The relevant lesson is that U.S. military support for allies embroiled in civil wars is critical to an acceptable political settlement — a truth that the Obama administration unfortunately has disregarded in Syria.

To be sure, Mr. Santos's prospective peace deal with the FARC remains controversial in Colombia. It is opposed by Mr. Uribe, who says terrorists who committed war crimes will escape consequential punishment and instead will be allowed to run for office. U.S. human rights groups raise similar objections about military commanders guilty of human rights abuses. Mr. Santos answers that all those guilty of crimes will be investigated, judged and sanctioned, though the penalties will not include conventional prison sentences or extradition to the United States.

As for future political activity by FARC leaders, that is one of the settlement’s virtues. “This is nothing different than improving our democracy,” Mr. Santos told us. “[Rural] regions that have never been represented in the political system will now be represented.”

To his credit, Mr. Santos committed to putting the final accord with the FARC to a referendum in Colombia, a risky vote given the widespread popular antipathy toward the group. If democratically ratified, the concessions made for peace will have a legitimacy that will merit support from Colombia’s allies, including the United States. The Obama administration has proposed increasing aid to Colombia to $450 million annually, while renaming the program Peace Colombia; that reinvestment in a U.S. success makes good sense.

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