Pakistan has had four constitutions since its independence in 1947. The 1956 constitution established Pakistan as an Islamic republic and parliamentary democracy. Martial law was imposed in 1958, replacing the 1956 constitution with General Ayub Khan's 1962 constitution, which established a presidential system. The 1973 constitution restored parliamentary democracy but was amended under Zia-ul-Haq to strengthen Islamization. It has since faced multiple amendments affecting the balance of power between civilian and military institutions.