The Sunday arrest of retired Venezuelan army colonel Oswaldo García Palomo by Venezuelan security forces is a setback for a nation struggling to unseat dictator Nicolás Maduro. But it is also a sign of the regime’s desperation.

National Assembly president Juan Guaidó took the oath as interim president last week as the Venezuelan constitution allows. Mr. Maduro has refused to step down but his own people and the free world are against him. Even the European Union, that paragon of diplomatic courage, has said it will recognize Mr. Guaidó if Mr. Maduro doesn’t agree to a new election by the weekend.

The regime claims it has the support of the armed forces. But if that were true Col. García wouldn’t be considered a threat. The former colonel hasn’t been heard from since his capture on the weekend. Much of the military opposes Mr. Maduro but fear deposing him under the watchful eye of Cuban intelligence.

Col. García is one of several dissidents who have been working to win over officers to the opposition. He has admitted to leading an effort last May to overthrow the dictatorship on the eve of the fraudulent presidential election. The plan was thwarted because the group was infiltrated. Col. García’s family members were arrested and tortured, according to a Caracas-based NGO. After they were released, Col. García took them out of the country. Mr. Maduro later called him “the boss of the assassins.”

The colonel continued to build support for Venezuelan democracy in exile and, judging by the international reaction to Mr. Guaidó, with significant success. His return to Venezuela was risky. Press reports say he was arrested in Barinas, the home state of Hugo Chávez and now a hotbed of opposition to the regime. Mr. Maduro may think his arrest weakens the opposition. Thirty million Venezuelans may see it as a sign of his weakness.