For the first time in at least a decade, Mexico's army is allowing the United States and the United Nations to observe opium poppy eradication, a step toward deeper cooperation to fight heroin traffickers, three sources in Mexico said. The opening could bring Mexico more in line with other drug producing countries like Afghanistan, Colombia and Peru that have been heavily involved with the United Nations in cultivation studies and eradication efforts. The Mexican army hopes to gain more credit at home and abroad for its work and address doubts in Washington about the quality of its data and the success of the eradication program, the officials said.