Vietnam: Cambodia bombing.

6,727,084 tons of bombs dropped on Indo China by the US during the Vietnam War. (from Vietnam Veterans Supporting Veterans xlnk.gif )
2,700,000 tons dropped by Allies in European theather
656,400 tons dropped by Allies in Pacific war
US Strategic Bombing Survey, 1945 xlnk.gif
2,756,941 tons of bombs dropped on Cambodia. Full-size map xlnk.gif Graphxlnk.gif

This only shows the bombing of Cambodia, not Vietnam and Laos.
Source: Bombs Over Cambodia xlnk.gif by Taylor Owen and Ben Kiernan, mapping by  Taylor Owen. Published in Walrus magazine (Canadian).