The Filipino constitution placed a two-term limit on the presidency. But, determined to stay in power beyond a second term, Marcos declared martial law in September 1972, imprisoning Aquino and other political opponents for their alleged involvement in a communist-inspired plot to overthrow the government. In November 1977, a military tribunal found Aquino guilty of subversion and sentenced him to death. Because of Aquino's international reputation, however, Marcos stayed the execution and in 1980 allowed Aquino to go into exile in the United States, where he would receive badly needed heart bypass surgery.
In 1981 Marcos lifted the martial law decree, and by 1983 his deteriorating health had weakened his grip on power. At that point, Aquino decided to return to the Philippines to work on the 1984 legislative elections and prepare for a post-Marcos return to democracy. Immediately upon his arrival at the Manila International Airport on 21 August 1983, Aquino was gunned down and murdered by a member of his own military escort, probably not on the order of the ailing Marcos but certainly on the command of someone else in his regime. Massive public demonstrations followed Aquino's assassination, as a growing number of Filipinos demanded an end to Marcos's brutal and corrupt rule.
In the wake of Aquino's murder, the administration of President Ronald Reagan, previously a strong Marcos proponent, began to withdraw support, calling for an investigation into the assassination, canceling a planned presidential trip to the Philippines, and urging free and fair elections. Although the United States never formally broke ties with Marcos, it allowed a popular rebellion to sweep him from power in February 1986 following his electoral defeat by Aquino's wife.
Thompson, Mark R. The Anti-Marcos Struggle: Personalistic Rule and Democratic Transition in the Philippines. New Haven, CT: Yale University Press, 1996.