During his presidency, Diệm strongly focused on his central concern: internal security to protect his regime as well as maintain order and social change: staunch anti-subversion and anti-rebellion policies. After the Bình Xuyên was defeated and the Hòa Hảo, Cao Đài were subdued, Diệm concentrated on his most serious threat: the communists. Diệm's main measures for internal security were threats, punishment and intimidation. According to Gabriel Kolko about 12,000 suspected opponents of Diệm were killed between 1955 and 1957 and by the end of 1958 an estimated 40,000 political prisoners had been jailed.