Between December 1956 and 1959, Castro led a guerrilla army against the forces of Batista from his base camp in the Sierra Maestra mountains. Batista's repression of revolutionaries had earned him widespread unpopularity, and by 1958 his armies were in retreat. On 31 December 1958, Batista resigned and fled into exile, taking with him an amassed fortune of more than US$300,000,000.
Fearing Castro was a socialist, the U.S. instructed Cantillo to oust Batista. By this time the great majority of Cuban people had turned against the Batista regime. Ambassador to Cuba, E. T. Smith, who felt the whole CIA mission had become too close to the MR-26-7 movement, personally went to Batista and informed him that the US no longer would support him and felt he no longer could control the situation in Cuba. General Cantillo secretly agreed to a ceasefire with Castro, promising that Batista would be tried as a war criminal; however, Batista was warned, and fled into exile with over US$300,000,000 on 31 December 1958.