fragging—attempting to kill unpopular officers and non-commissioned officers with grenades or other weapons—created severe problems for the U.S. military and impacted its capability of undertaking combat operations. Between 1969 and 1971 the U.S. Army recorded more than 900 attacks by troops on their own officers and NCOs with 99 killed. An estimated 125,000 Americans left for Canada to avoid the Vietnam draft, and approximately 50,000 American servicemen deserted. In 1977, United States president Jimmy Carter granted a full and unconditional pardon to all Vietnam-era draft dodgers with Proclamation 4483. The Vietnam War called into question the U.S. Army doctrine. Marine Corps general Victor H. Krulak heavily criticised Westmoreland's attrition strategy, calling it "wasteful of American lives... with small likelihood of a successful outcome." In addition, doubts surfaced about the ability of the military to train foreign forces. Furthermore, throughout the war there was found to be considerable flaws and dishonesty by officers and commanders due to promotions being tied to the body count system touted by Westmoreland and McNamara. And behind the scenes Secretary of Defense McNamara wrote in a memo to President Johnson his doubts about the war: "The picture of the world's greatest superpower killing or seriously injuring 1,000 noncombatants a week, while trying to pound a tiny backward nation into submission on an issue whose merits are hotly disputed, is not a pretty one." Effects of U.S. chemical defoliation One of the most controversial aspects of the U.S. military effort in Southeast Asia was the widespread use of chemical defoliants between 1961 and 1971. 20 million gallons of toxic herbicides (like Agent Orange) were sprayed on 6 million acres of forests and crops by the U.S. Air Force. They were used to defoliate large parts of the countryside to prevent the Viet Cong from being able to hide weaponry and encampments under the foliage, and to deprive them of food. Defoliation was also used to clear sensitive areas, including base perimeters and possible ambush sites along roads and canals. More than 20% of South Vietnam's forests and 3.2% of its cultivated land was sprayed at least once. 90% of herbicide use was directed at forest defoliation. The chemicals used continue to change the landscape, cause diseases and birth defects, and poison the food chain. Agent Orange and other similar chemical substances used by the U.S. have also caused a considerable number of deaths and injuries in the