The most important parallel, though, isn't legal or moral: it's strategic. As critics wonder what kind of backlash might ensue from drone attacks that kill civilians and terrorize communities, Cambodia provides a telling historical precedent.

Between 1965 and 1973, the U.S. dropped 2.7 million tons of explosives -- more than the Allies dropped in the entirety of World War II -- on Cambodia, whose population was then smaller than New York City's. Estimates of the number of people killed begin in the low hundreds of thousands and range up from there, but the truth is that no one has any idea.

The bombing had two primary effects on survivors. First, hundreds of thousands of villagers fled towards the safety of the capital Phnom Penh, de-stabilizing Cambodia's urban-rural balance. By the end of the war, the country's delicate food supply system was upended, and the capital was so overcrowded that residents were eating bark off of trees.

Secondly, the attacks radicalized a population that had previously been neutral in the country's politics. The severity of the advanced air campaign -- "I want everything that can fly to go in there and crack the hell out of them," then-U.S. President Richard Nixon told National Security Adviser Henry Kissinger -- fomented immense anger in the Cambodian countryside. Charles Meyer, an aide to the deposed Prince Sihanouk, said that it was "difficult to imagine the intensity of [the peasants'] hatred towards those who are destroying their villages and property." Journalist Richard Dudman was more precise. "The bombing and the shooting," he wrote after a period in captivity in the Cambodian jungle, "was radicalizing the people of rural Cambodia and was turning the country into a massive, dedicated, and effective rural base."

Nevertheless, many historians continued to deny the causal link between the violence and the political upheavals in the country. Cambodia's embrace of radicalism instead fit neatly into the Cold War-era "domino theory" paradigm, de-emphasizing the role of local conditions in driving the country's history.

William Shawcross, in 1979's Sideshow: Kissenger. Nixon and the Destruction of Cambodia , was the first to advance the theory that the meteoric rise of the Khmer Rouge was not in spite of the U.S. bombing campaign but because of it. Taylor Owen, the research director of the Tow Center for Digital Journalism at Columbia, and Ben Kiernan, director of the Genocide Studies Program at Yale, have concluded that the full war archives, released by President Clinton in 2000, confirm this version of history.

"The impact of this bombing... is clearer than ever," they write. "Civilian casualties in Cambodia drove an enraged populace into the arms of an insurgency that had enjoyed relatively little support until the bombing began, setting in motion the expansion of the Vietnam War deeper into Cambodia, a coup d'etat in 1970, the rapid rise of the Khmer Rouge, and ultimately the Cambodian genocide."