When Taft entered office, Mexico was increasingly restless under the grip of longtime dictator Porfirio Díaz. Many Mexicans backed his opponent, Francisco Madero. There were a number of incidents in which Mexican rebels crossed the U.S. border to obtain horses and weapons; Taft sought to prevent this by ordering the US Army to the border areas for maneuvers. Taft told his military aide, Archibald Butt, that "I am going to sit on the lid and it will take a great deal to pry me off". He showed his support for Díaz by meeting with him at El Paso, Texas, and Ciudad Juárez, Mexico, the first meeting between a U.S. and a Mexican president and also the first time an American president visited Mexico. The day of the summit, Frederick Russell Burnham and a Texas Ranger captured and disarmed an assassin holding a palm pistol only a few feet from the two presidents. Before the election in Mexico, Díaz jailed opposition candidate Madero, whose supporters took up arms. This resulted in both the ousting of Díaz and a revolution that would continue for another ten years. In the U.S.'s Arizona Territory, two citizens were killed and almost a dozen injured, some as a result of gunfire across the border. Taft was against an aggressive response and so instructed the territorial governor.