at the beginning of june, on the heels of their success at ba Gia, the Communists launched a series of major attacks in the highlands of northern II Corps and in Quang Ngai province. The 325th North Vietnamese Army Division and the 320th North Vietnamese Army Regiment participated heavily in these operations, sallying from highland bases to strike government forces and centers of power. The highlands offered the Communists critical advantages, both as bases and as operational areas. Hanoi could supply the highlands relatively easily, via the Ho Chi Minh Trail, because of their proximity to Laos and North Vietnam. The Saigon government's trucks and armored personnel carriers could not traverse many of the mountainous highland areas, removing the advantages in troop mobility, logistics, and reconnaissance that these vehicles typically conferred upon the government. In order to compound the advantages of the highlands, the Communists had deliberately chosen to strike during the monsoon season. From mid-May to mid-October, the southwest monsoon relentlessly dumped rain on the central highlands, turning many of the region's dirt roads into mud and rendering them impassable to South Vietnamese trucks. The low clouds and fog endemic to the monsoon season often obscured the peaks of hills and mountains, which made the flying of aircraft prohibitively dangerous. The strategically vital highland province of Pleiku, for example, enjoyed an average of twenty-five days of good flying weather per month during the dry season but had only six days per month in the monsoon season.