Between May 1964 and the summer of 1969, the Plain of Jars was heavily bombed by the U.S. Air Force (USAF) (see Secret War) operating against North Vietnamese and Pathet Lao communist forces. The USAF dropped more bombs on Laos, primarily the Plain of Jars, than it dropped during the whole of World War II. This included 262 million anti-personnel cluster bombs. An estimated 80 million of these did not explode and remain a deadly threat to the population.