The correct answer is the Vietnam War.
On the home front, President Lyndon B. Johnson was dedicated to his Great Society programs. This included an expansion of Medicare and Medicaid, declaring a "War on poverty," and improving education. However, America's dedication to the foreign policy of containment decreased the amount of money available for these programs.
The Vietnam War saw the North Vietnamese fighting against the South Vietnamese. The US supported the South Vietnamese, as the North Vietnamese were communist. During this time, the US was trying to stop the spread of communism (also known as containment). Thanks to the increased amount of troops and money the US was using to help South Vietnam, there was not enough funds leftover to adequately fund all the Great Society programs.