It’s a safe bet to say most people outgrew their fascination with playing in mud sometime in elementary school. Perhaps even fewer would consider it a likely weapon of war.

But in the 1960s, the United States began to look seriously at mud—yes, mud—as a way of preventing the North Vietnamese from sneaking soldiers and equipment into South Vietnam.

The story begins following the 1954 Geneva Accords, which divided Vietnam into two states. The North Vietnamese government sought to reunify the country under communist leadership, and proceeded to arm and supply a guerrilla movement in South Vietnam. To supply the insurgents, the North constructed a massive network of infiltration routes on land and at sea.

The land route, eventually nicknamed the Ho Chi Minh trail, grew to hundreds of miles of roads, laboriously cut into the jungle through neighboring Laos and Cambodia. In some areas, the trail was built up to semi-improved highways crawling with Soviet and Chinese-supplied trucks.

Every day—at its height—hundreds of tons of material made its way south. And for the Pentagon, stopping this flow of supplies was critical to blunting a growing anti-U.S. insurgency.

With Laos technically neutral, the U.S. focused on air strikes. But seeing trucks under triple-canopy jungle was difficult, to say the least. “Secondary explosions” were often cited as evidence—speculation, at best—of hitting something. Targeting air strikes against bicycles or people under the same conditions was basically impossible.

Physical barriers, acoustic sensors, miniature landmines and self-destructing caltrops were among the numerous ideas tested in the field. Others never made it past the development phase.

Yet mud—wet, sloppy mud—emerged as another option.

The U.S. was no stranger to the effect that rain could have on logistics. Mud was one of the many issues that the famed Red Ball Express had contended with as its trucks sped across Europe during World War II. In the 1952 dramatic movie on the subject, The Red Ball Express, the narrator describes the mud following eight days of rain as consisting of “two parts glue and one part perversity.”

It was with such perversity in mind that the Pentagon began Project Popeye in 1966, an attempt to artificially extend the monsoon season in Southeast Asia. Between 1966 and 1972, Air Weather Service WC-130s flew over Laos, dispensing crushed, dried ice in hopes of creating floods along the trail and in agricultural areas of North Vietnam.

But this early step didn't work. No conclusive evidence was ever offered that the program was responsible for any significant changes in annual rainfall or the duration of the monsoon season. The program cost millions of dollars, and when it became public knowledge, was met with considerable criticism from Congress and the public.