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U.S.-Mexico security cooperation has been strikingly close and effective during the tenure of Mexican President Felipe Calderón. A country that had traditionally seen the United States as the principal threat to its national security has come to accept its northern neighbor as a partner in the battle against organized crime. Mexican intelligence agencies and naval units now collaborate closely with U.S. security personnel despite the historic reluctance of Mexico’s highly nationalistic military establishment to do so. At the same time, the United States, a country that had traditionally seen Mexico as a weak and unreliable counterpart, has learned to see its southern neighbor as an increasingly trusted associate. The United States now willingly shares sensitive intelligence with...