Operation Popeye (Project Controlled Weather Popeye / Motorpool / Intermediary-Compatriot) was a military cloud-seeding project carried out by the U.S. Air Force during the Vietnam War in 1967–1972. The highly classified program attempted to extend the monsoon season over specific areas of the Ho Chi Minh Trail, to disrupt North Vietnamese military supplies by softening road surfaces and causing landslides.
The chemical weather modification program was conducted from Thailand over Cambodia, Laos, and Vietnam and allegedly sponsored by Secretary of State Henry Kissinger and the CIA without the authorization of then Secretary of Defense Melvin Laird, who had categorically denied to Congress that a program for modification of the weather for use as a tactical weapon even existed
https://en.wikipedia.org/wiki/Operation_Popeye 