Vietnam War

Cambodia- Between 1965-1973, the US Air Force secretly dropped tons of bombs, and other ordinance including land mines, experimental weapons and rockets.

The campaign's aim was to expose the Ho Chi Minh Trail used by Communist North Vietnamese troops and to support the pro-American Lon Nol government fight the communist Khmer Rouge. The bombing of Cambodian villages and its significan civilian casualties provide ideal recruitment rhetoric for the insurgent Khmer Rouge.
The figure exceeds the 2 millions tons of bombs dropped in the whole of World War II, including the two nuclear bombs.
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Bombs Over Cambodia: New Light on US Air War by Taylor Owen and Ben Kiernan (Japan Focus; May 12, 2007)