Operation Proud Deep was a five-day limited air campaign commenced on 26 December 1971 against specified North Vietnamese targets including fuel and supply depots, airfields, SAM sites and truck parks below the 20th parallel. Ostensibly the offensive was mounted in response to the build up of North Vietnamese SAM sites and MiG activity in the southern provinces of the North. The campaign was to consist of no more than about 1,000 bombing sorties but it got off to a bad start due to bad weather caused by the northeast monsoon. Due to the low cloud, most aircraft had to drop their bombs on instruments with correspondingly poor results.