The U.S. bombing of Laos (1964-1973) was part of a covert attempt by the CIA to wrest power from the communist Pathet Lao, a group allied with North Vietnam and the Soviet Union during the Vietnam War.

The officially neutral country became a battleground in the Cold War between the United States and Soviet Union, with American bombers dropping over two million tons of cluster bombs over Laos—more than all the bombs dropped during WWII combined. Today, Laos is the most heavily bombed nation in history. Here are facts about the so-called secret war in Laos.

Laos is a landlocked country bordered by China and Myanmar to the North, Vietnam to the East, Cambodia to the South and Thailand and the Mekong River to the West.

Its proximity to Mao Zedong’s China made it critical to Dwight D. Eisenhower’s Domino Theory of keeping communism at bay. “If Laos were lost, the rest of Southeast Asia would follow,” Eisenhower told his National Security Council. On the day of his farewell address in 1961, President Eisenhower approved the CIA’s training of anti-communist forces in the mountains of Laos. Their mission: To disrupt communist supply routes across the Ho Chi Minh Trail to Vietnam.

Eisenhower’s successors in the White House: John F. Kennedy, Lyndon B. Johnson and Richard Nixon, all approved escalating air support for the guerrilla fighters, but not publicly. The 1962 International Agreement on the Neutrality of Laos, signed by China, the Soviet Union, Vietnam, the United States and 10 other countries, forbid signees from directly invading Laos or establishing military bases there. The secret war in Laos had begun.

Long before the Cold War, Laos had a history of interference from its neighbors. Fa Ngum founded the first recorded Lao state of “Lan Xang,” or “The Kingdom of a Million Elephants,” in 1353. From 1353-1371, Fa Ngum went on to conquer most of today’s Laos and parts of what is now Vietnam and Northeast Thailand, bringing Theravada Buddhism and Khmer culture from the kingdom of Angkor (in today’s Cambodia) with him.

Over the centuries, his conquered neighbors fought back, and the Thai people dominated large swaths of Laos from the late 1700s to the early 1800s. What we know as Laos today was built from an assemblage of different ethnic groups with distinct languages and cultures.

Europeans entered the fray in 1893, when France declared Laos part of French Indochina. To the French, having Laos as a protectorate was a means to control the Mekong River, a valuable trade route through Southeast Asia.

France’s grasp on Laos first slipped in 1945, when the Japanese occupied Laos in the closing days of World War II. When atomic bombs fell on Japan, Laos declared its independence under the short-lived Lao Issara (“Free Laos”) government of Prince Phetsarath in 1945. The French regained power the following year.

Laos achieved full independence in 1954 following the victory of communist Việt Minh leader Ho Chi Minh over the French at the bloody Battle of Điện Biên Phủ. The ensuing Geneva Accords split Vietnam into North and South Vietnam and stipulated that the French relinquish their claims in Southeast Asia. The agreement was not signed by the United States, who feared that in the absence of French influence, Southeast Asia would fall to communist forces.

The United States watched closely as the Pathet Lao gained popularity in newly-independent Laos. The Pathet Lao was a communist group founded at Viet Minh headquarters in 1950 during the French war. Largely dependent on Vietnamese aid, their leader was Prince Souphanouvong, the “Red Prince.” Born to a prince of Luang Prabang and a commoner, his education in Vietnam led him to become a disciple of Ho Chi Minh and, later, to lead the opposition against his half-brother, Souvanna Phouma, who was Prime Minister of Laos five different times (from 1951-1954, 1957-1958, in 1960 and again from 1962-1972) and preferred a coalition government balancing the Pathet Lao with more conservative forces.

Phouma’s hold on power was tenuous at best. Under his rule, government troops and the Pathet Lao began to clash in the Northeast along the border of Vietnam. Publicly, President Kennedy announced his support for neutralizing Laos—though what neutralization looked like on paper was far different from what it was in practice.

In 1960, the CIA approached Vang Pao, a major general in the Royal Lao Army and a member of the Hmong minority in Laos, to be the chief of their secret army to push back the communist Pathet Lao. The Hmong made up an ethnic group that had originated in China and lived in the remote mountains of Laos, often in extreme poverty, and had a history of evading authority. They had been at odds with the lowland Lao majority for centuries, and the CIA exploited this history of conflict to their benefit.

Charismatic and prone to pacing while he talked, Vang Pao had experience fighting both the French and the Japanese. His followers praised him for his bravery in fighting alongside his men. The CIA’s Operation Momentum armed and trained the Hmong to take on the Pathet Lao in the growing proxy war.

A ground war in Laos with U.S. forces was not on the table. President Kennedy wrote as early as 1961 that, “Laos…is a most inhospitable area in which to wage a campaign. Its geography, topography, and climate are built-in liabilities.” Bombing Laos was seen as a safer means of cutting off communist supply lines into Vietnam before they could be used against American troops.

The U.S. Air Force began bombing targets in Laos in 1964, flying planes like AC-130s and B-52s full of cluster bombs on covert missions based out of Thailand. The United States eventually dropped the equivalent of a planeload of bombs every eight minutes, 24 hours a day, for nine years, according to Al Jazeera.

The bombing focused on disrupting communist supply chains on the Ho Chi Minh Trail and Sepon (also spelled Xépôn), a village near a former French air base now controlled by North Vietnam. In 1971, Sepon was the target of the failed Operation Lam Son, when the U.S. and South Vietnam attempted to block access to the Ho Chi Minh Trail.

Dave Burns, a member of the U.S. Air Force’s 16 Special Operations Squadron, flew missions over Laos out of Ubon, Thailand. He recalls, “Sepon was the one place in Laos that we did not want to fly into. The village was at a crossroads of three highways leading in from Vietnam: the Mu Gia Pass, the Ban Karai Pass, and the Barthelme Pass. The highways then headed south to the Ho Chi Minh Trail. It was highly defended with all sorts of anti-aircraft guns. Going there was a guarantee of being hit or being shot down.”

Air America was the lifeblood of the CIA’s Laos operation, transporting personnel, food and supplies to and from remote bases. As a former CIA officer explained: “We’d negotiate with the tribal groups. If you don’t make a deal with them, give them aid, the communists will do it, and then they’ll join with the communists.” The CIA set up medical facilities with doctors, started schools and offered protection from rivals.

Air America was also transporting more illicit goods. In the 1979 book Air America by Christopher Robbins, later immortalized in the fictional "Air America" movie starring Mel Gibson and Robert Downey, Jr., Robbins reports on how opium from Lao poppies was transported on American planes.

By 1975, one-tenth of the population of Laos, or 200,000 civilians and members of the military, were dead. Twice as many were wounded. Seven hundred and fifty thousand, a full quarter of the population, had become refugees—including General Vang Pao himself. Declassified documents show that 728 Americans died in Laos, most of whom were working for the CIA. The secret war in Laos, or the Laos Civil War to many who lived through it, set a precedent for a more militarized CIA with the power to engage in covert conflicts around the world.

In Laos, the legacy of U.S. bombs continues to wreak havoc. Since 1964, more than 50,000 Lao have been killed or injured by U.S. bombs, 98 percent of them civilians. An estimated 30 percent of the bombs dropped on Laos failed to explode upon impact, and in the years since the bombing ended, 20,000 people have been killed or maimed by the estimated 80 million bombs left behind.

In 2016, President Barack Obama became the first sitting U.S. president to visit Laos. He pledged an additional $90 million in aid to remove unexploded ordnance on top of the $100 million that had been spent previously. The work of cleaning out unexploded bombs from the soil continues.