In 1966, the idea of raining on Vietnam became the top-secret Project Popeye, which ran for some seven years and included more than 2,600 cloud-seeding flights over Vietnam and Laos. The objective was simple: Make rain that would make or keep the Ho Chi Minh Trail—a main supply route for the North Vietnamese—so muddy that it was unusable. (Why "Popeye"? The artificially created rain was apparently called "Olive Oil.")
The story of the top-secret project—flown by the Air Force but controlled by the CIA and the White House—was broken in 1971 in Jack Anderson's national newspaper column, then, to greater fanfare, in July 1972, with Hersh's front-page story in The New York Times. And while there were no rules at the time about weather modification—or any environmental warfare, for that matter—the Nixon administration was not happy with Hersh's revelations. The White House and the State Department declined comment, and one unnamed official said, "This is one of those things where no one is going to say anything."6 (People said even less about CIA weather-modification in Cuba. During 1969 and 1970, planes from China Lake seeded clouds that rained over non-agricultural regions of Cuba, leaving at least some of the country's sugar cane fields dry.)
The eventual response was an international treaty, the Convention on the Publication of Military or Any Other Hostile Use of Environmental Modification Techniques, or ENMOD, which was ratified by the US in 1977. The main tenet of the treaty, which stands today, is this: "Each State Party to this Convention undertakes not to engage in military or any other hostile use of environmental modification techniques having widespread, long-lasting or severe effects as the means of destruction, damage or injury to any other State Party."7
Weather-control activities seem to have quieted down after ENMOD, but fortunately for the US military, the treaty has a loophole big enough to drive a truck through: It prohibits only action with "widespread, long-lasting or severe effects." When Air Force intelligence analysts turned their thoughts to the weather in the mid-1990s, they made up their own definition of these limits: widespread means affecting more than several hundred kilometers; long-lasting means for a period of months; and severe "involves serious or significant disruption or harm to human life, natural or economic resources, or other assets."8 Which is to say that, except for the human life part, most military applications, which tend to be short-term and localized, are allowed. These analysts' conclusions are contained in an extraordinary report, published in 1996, called "Weather as a Force Multiplier: Owning the Weather in 2025." If the US intelligence community did ignore the weather for a while, "Owning the Weather" certainly made up for it. Envisioning a world where technology has made control over the local weather phenomena much more precise—and where a global network of sensors has greatly increased available atmospheric data—the writers declare that in three decades time, "US aerospace forces can 'own the weather'," and "shape the battlespace in ways never before possible."
(I only found this in a cache on google)