This paper responds to a previous Strategic Forum (no. 243, July 2009) entitled U.S.-Mexico Defense Relations: An Incompatible Interface by Craig Deare. Some of the assertions and conclusions within Dr. Deare's paper were flawed due to an outdated U.S.-Mexico paradigm that preceded the 9/11 attacks and recent counterdrug operations in Mexico. If his work had been published prior to the establishment of U.S. Northern Command (USNORTHCOM), would have been well received, but times have changed. Because of our collective experiences over the past 6 years, we find implausible the notion that USNORTHCOM is not staffed or experienced enough to support Mexico's security cooperation needs. Hence, U.S.-Mexico Homeland Defense: A Compatible Interface is intended to set the record straight by pointing out the numerous areas of cooperation between Mexico and the United States since the establishment of USNORTHCOM. The term compatible interface in the title of this paper refers to the fact that the United States and Mexico have complementary areas of concern in each of our four instruments of national power: diplomatic, information, military, and economic. A firm understanding of where we were, and where we are, in these elements of power gives the reader a better picture of the strong relationship between Mexico and the United States.