Lansdale believed the United States could win guerrilla wars by studying the enemy's psychology, an approach that won the approval of the presidential administrations of both John F. Kennedy and Lyndon B. Johnson.

Lansdale struggled in learning foreign languages while at UCLA and told a biographer he retained almost nothing except Spanish curse words and French phrases he had memorized in kindergarten.

[2] Lansdale's difficulty meeting the foreign language requirement for his degree and lack of obvious career path prompted him to drop out of UCLA several credits short of graduation.

After leaving the Philippines in 1948, he served as an instructor at the Strategic Intelligence School at Lowry Air Force Base, Colorado, where he received a temporary promotion to lieutenant colonel in 1949.

[8] Lansdale helped the Philippine Armed Forces develop psychological operations, civic actions, and the rehabilitation of Hukbalahap prisoners.

"[9] Lansdale had previously been a member of General John W. O'Daniel's mission to Indochina in 1953, acting as an advisor to French forces on special counter-guerrilla operations against the Viet Minh.

[4] During this period, he was active in the training of the Vietnamese National Army (VNA), organizing the Caodaist militias under Trình Minh Thế in an attempt to bolster the VNA, a propaganda campaign encouraging Vietnam's Catholics to move to the south as part of Operation Passage to Freedom, and spreading claims that North Vietnamese agents were making attacks in South Vietnam.

[9] In October 1954, Lansdale foiled a coup attempt, cutting General Nguyễn Văn Hinh's communication off from his top lieutenants by moving them to Manila.

Lansdale mentored and trained Phạm Xuân Ẩn, a reporter for Time magazine who was actually a highly placed North Vietnamese spy.

In 1961, he helped to publicize the story of Father Nguyễn Lạc Hoá, the "fighting priest" who had organized a crack militia, the Sea Swallows, from his village of anticommunist Chinese Catholic exiles.

During the early 1960s, he was chiefly involved in clandestine efforts to topple the government of Cuba, including proposals to assassinate Fidel Castro.