Although Cooper voted in favor of the 1964 Gulf of Tonkin Resolution, he opposed escalating U.S. involvement in the Vietnam War. As early as April 1964, Cooper was urging President Johnson to negotiate a peaceful settlement to the tensions in Southeast Asia. He questioned Southeast Asia's strategic importance to the U.S. and expressed concerns about the feasibility of deploying the U.S. military on a global scale. On March 25, 1965, he joined New York Senator Jacob Javits in a call for President Johnson to begin negotiations for a settlement between North Vietnam and South Vietnam without imposing preconditions on the negotiations. Later in the day, he introduced resolutions calling for Secretary of State Dean Rusk and Secretary of Defense Robert McNamara to brief the full Senate on recent developments in Vietnam.