JFK was assassinated on November 22, 1963, as he approached the end of his third year in office. Researchers who spent years studying the evidence have concluded that President Kennedy was assassinated by a conspiracy between the CIA, Joint Chiefs of Staff, and Secret Service. (See, for example, JFK and the Unspeakable by James W. Douglass)

Kennedy entered office as a cold warrior, but he learned from his interaction with the CIA and Joint Chiefs that the military/security complex had an agenda that was self-interested and a danger to humanity. He began working to defuse tensions with the Soviet Union. His rejections of plans to invade Cuba, of the Northwoods project, of a preemptive nuclear attack on the Soviet Union, and his intention to withdraw from Vietnam after his reelection, together with some of his speeches signaling a new approach to foreign policy in the nuclear age (see for example, https://www.jfklibrary.org/Asset-Viewer/BWC7I4C9QUmLG9J6I8oy8w.aspx ), convinced the military/security complex that he was a threat to their interests. Cold War conservatives regarded him as naive about the Soviet Threat and a liability to US national security. These were the reasons for his assassination. These views were set in stone when Kennedy announced on June 10, 1963, negotiations with the Soviets toward a nuclear test ban treaty and a halt to US atmospheric nuclear tests.