During the Vietnam War, highly classified attempts were made to modify weather over North Vietnam; the program was most often called Operation POPEYE (also “Intermediary-Compatriot.”). The main motivation was to damage North Vietnamese logistics by increasing rain and turning dirt roads into seas of mud, and, ideally, to cause mudslides that could close the passes between China and North Vietnam.
Weather warfare took a macro-pathological turn between 1967 and '72 in the jungles over North and South Vietnam, Laos, and Cambodia. Using technology developed at the naval weapons testing center at China Lake, California, to seed clouds by means of silver iodide flares, the military conducted secret operations intended, among other goals, to "reduce trafficability" along portions of the Ho Chi Minh Trail, which Hanoi used to move men and material to South Vietnam. Flights using silver iodide flares were flown by the Air Weather Service from Udorn Air Base in Thailand. Knowledge of the operation was tightly held, but was approved by Presidents Lyndon B. Johnson and Richard M. Nixon Over 2,600 cloud seeding sorties were flown between 1967 and 1971, expending 47,000 silver iodide flares over a period of approximately five years.
In 1967, the missions were first flown by three WC-130A weather aircraft belonging to the 54th Weather Reconnaissance Squadron at Udorn; two RF-4C reconnaissance jets also participated. President Johnson was briefed on the technique by U.S. Navy Captain Sam Houston and approved. It was considered extremely sensitive due to potential treaty violations, and only a few operational personnel were aware of it. Columnist Jack Anderson revealed it in 1971.
Some defended the use of environmental weapons, arguing that they were more “humane” than nuclear weapons. Others suggested that inducing rainfall to reduce trafficability was preferable to dropping napalm. As one wag put it, “Make mud, not war.” At a congressional briefing in 1974, military officials downplayed the impact of Operation POPEYE, since the most that could be claimed were 10 percent increases in local rainfall, and even that result was “unverifiable.” Philip Handler, president of the National Academy of Sciences, represented the mainstream of scientific opinion when he observed, “It is grotesquely immoral that scientific understanding and technological capabilities developed for human welfare to protect the public health, enhance agricultural productivity, and minimize the natural violence of large storms should be so distorted as to become weapons of war.”
An unexpected consequence may have been to cause flooding which led to the evacuation of the military prison at Son Tay, which U.S. special operations forces raided, in Operation IVORY COAST, on 21 November 1970, finding no prisoners of war. 
Subsequently, such actions were forbidden by the UN Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD), which entered into force in 1978, ratified by 70 nations.
The U.S. Senate, when Anderson broke the news, became concerned and investigated. Sen. Claiborne Pell asked the Defense Department for information on On 23 September 1971,. After waiting 4 months to answer, the Defense Department declined to reply on the basis that it would threaten national security.  Testifying to the Senate on 18 April 1972, Secretary of Defense Melvin Laird denied any weather modification in Northern Vietnam, saying "we have never engaged in that type of activity over Northern Vietnam."
Pell introduced a resolution "expressing the sense of the Senate that the US Government should seek the agreement of other governments to a proposed treaty prohibiting any use of an environmental or geophysical modification activity as a weapon of war, or the carrying out of any research or experimentation with respect thereto." It was approved a year later as Resolution 71, on 11 July 1973.
- Hank Bradli, The Use of Weather Satellite Photos in Vietnam War
- James R. Fleming, "The Climate Engineers", The Wilson Quarterly, Woodrow Wilson International Center for Scholars
- Benjamin Schlemmer, The Raid
- US Senate, Subcommittee on Oceans and International Environment; 26 July 1972; p. 4
- The Limits of Inside Pressure: The US Congress Role in ENMOD