 The balance of power shifted decisively in North Vietnam's favor, and the North subsequently launched a major military offensive, the Ho Chi Minh Campaign, against the South that culminated in the surrender of the Republic of Vietnam to PAVN forces on April 30, 1975.The earliest reported use of drugs among US troops in Vietnam was recorded in 1963.During this time the most commonly used drug was marijuana, which was sometimes used in the form of hashish.