Operation Popeye was a secretive U.S. military weather modification program conducted during the Vietnam War from March 20, 1967, to July 5, 1972. Its primary objective was to disrupt North Vietnamese supply lines along the Ho Chi Minh Trail by extending the monsoon season in Laos, Vietnam, and Cambodia. By employing cloud seeding—dispersing silver iodide or lead iodide into clouds to stimulate rainfall—the operation aimed to make roads impassable, cause flooding, and hinder enemy troop and supply movements. Proposed by the Joint Chiefs of Staff in 1966, the program was executed by the 54th Weather Reconnaissance Squadron, primarily using C-130 aircraft based in Thailand. It cost approximately $3.6 million per year (about $23 million in 2025 dollars).The operation stemmed from earlier experiments, including a 1966 test in Laos that successfully increased rainfall. Over its five-year run, Popeye conducted thousands of seeding missions, with estimates suggesting an additional 35 inches of rainfall in targeted areas. However, assessments of its effectiveness were mixed; while it caused localized flooding and muddied trails, it did not significantly disrupt North Vietnamese operations, as the enemy adapted by improving logistics and infrastructure. The program was highly classified, known only to select military and government officials, until its exposure through leaks, including the Pentagon Papers, and a 1974 Senate briefing led by Senators Claiborne Pell and Clifford Case.Public revelation sparked widespread controversy over the ethics of weaponizing weather, contributing to the 1976 Environmental Modification Convention (ENMOD), ratified in 1978, which banned hostile weather modification.

Subsequent declassified records, including those from 1974 Senate hearings , include statements from Air Force personnel involved in the operation. Pilots and crew members of the 54th Weather Reconnaissance Squadron described their missions as routine but highly secretive. One unnamed pilot, quoted in declassified reports, noted the challenge of flying into monsoon clouds to release silver iodide flares, describing it as "like flying into a washing machine" due to turbulent conditions. Crew members were often unaware of the broader strategic goals, briefed only on technical aspects of cloud seeding.