Operation Popeye (Project Popeye / Motorpool / Intermediary-Compatriot ) was a US military cloud seeding operation (running from March 20, 1967 until July 5, 1972) during the Vietnam war to extend the monsoon season over Laos, specifically areas of the Ho Chi Minh Trail. The operation seeded clouds with silver iodide, resulting in the targeted areas seeing an extension of the monsoon period an average of 30 to 45 days. As the continuous rainfall slowed down the truck traffic, it was considered relatively successful.[1] The 54th Weather Reconnaissance Squadron carried out the operation to "make mud, not war."
http://en.wikipedia.org/wiki/Operation_Popeye

WeaponisationThe Environmental Modification Convention generally prohibits weaponising geoengineering techniques. However, this does not eliminate the risk. Geoengineering techniques may serve as weapons of mass destruction, creating droughts or famines designed to destroy or disable an enemy.[68] They could also be used simply to make battlefield conditions more favourable to one side or the other in a war[69] (such as in Operation Popeye). For example, laser-guided weapons are confounded by clouds, and thus switching off cloud machines would favour forces using such weapons, and switching them on would favour ground forces defending against them.[70]

Whilst laws or treaties may prevent the manipulation of the climate as a weapon of war,[71] it could be argued that geoengineering is itself a manipulation, and thus destroying or disabling the geoengineering structures is not prohibited. A new legal framework may be necessary in the event that large-scale geoengineering becomes established.[original research?]

Carnegie’s Ken Caldeira said, "It will make it harder to achieve broad consensus on developing and governing these technologies if there is suspicion that gaining military advantage is an underlying motivation for its development..."[72]
http://en.wikipedia.org/wiki/Geoengineering#Weaponisation