It was crunch time, and that's when the U.S. government rolled out one of their wildest cards: "Operation Popeye." What was it? A secret plan to turn the weather into a weapon to make it rain and flood out the Ho Chi Minh Trail — the Viet Cong's main thoroughfare.

Popular Science:

Though it cycled through several names in its history, "Operation Popeye" stuck. Its stated objective—to ensure Americans won the Vietnam War—was never realized, but the revelation that the U.S. government played God with weather-altering warfare changed history. The Nixon administration distracted, denied, and, it seems, outright lied to Congress, but enterprising reporters published damning stories about rain being used as a weapon, and the Pentagon papers dripped classified details like artificial rain. Eventually, the federal government would declassify its Popeye documents and international laws aimed at preventing similar projects would be on the books.

But the public would, more or less, forget it ever happened. Given the rise of geo-engineering projects, both from municipal governments and private companies, some experts believe Popeye is newly relevant.

Operation Popeye relied on a technique called "cloud seeding," where crystals are dropped into clouds to induce rainfall.

The Popular Science article continues:

At first, no one seemed to consider the wartime applications of cloud seeding, but on March 20, 1967, the "operational phase" of Popeye began. Pilots and their crew would soar over select regions of Vietnam with a canister of silver or lead iodide, which were, by the 1960s, considered two of the primary sources of water condensation nuclei. The plane crew would ignite the canisters and release particle-rich smoke into an existing storm. If all went well, the jolt of artificial nuclei would reverberate through the system, forcibly spurring additional precipitation.