Contrary to popular belief, the decision to cap U.S. troop levels in Vietnam came months before the Tet Offensive of 1968.
On April 27, 1967, General William C. Westmoreland met in the White House with President Lyndon Baines Johnson to discuss future American troop levels in South Vietnam. During the course of their discussions, Westmoreland told the president that the current authorized troop level of 470,000 men, scheduled for deployment by the end of 1967, would not be enough to attain U.S. goals in South Vietnam. Specifically, those troops would neither bleed the enemy dry nor break his will to resist. Westmoreland noted, “With the present program of 470,000 men, we would be setting up a meat grinder [in South Vietnam]. We would do a little better than hold our own.” In other words, with almost half a million U.S. troops in South Vietnam, the U.S. risked being stuck in a ground war that showed little progress or might even devolve into an indecisive stalemate.
In order to prevent a stalemate and shorten the war, Westmoreland wanted reinforcements. He informed the president that with an additional 95,000 troops he could bring the war to a successful conclusion in three years. Yet, even with those extra troops, “…we will not be in danger of being defeated, but it will be nip and tuck to oppose the reinforcements the enemy is capable of providing.” Westmoreland told Johnson that if he had an additional 195,000 troops (on top of the 470,000 already programmed for South Vietnam), he could end the war in two years. Oddly, Westmoreland did not explain to the president the basis for his conclusions. He neither presented evidence nor analysis to support his assertion that more troops would end the war sooner on terms favourable to the United States.
At this meeting, Westmoreland made the same argument to the president that he had made back in mid-1966 – he had to have reinforcements to make his search and destroy strategy work. Back then the president had given him the troops. This time, Johnson remained noncommittal. Westmoreland finished his meeting with Johnson by urging the president to grant his request for an additional 195,000 troops as soon as possible. He wanted the first 100,000 immediately and the second increment no later than July 1, 1968. The longer he went without the reinforcements, the longer the war would drag on.
Westmoreland’s request for additional reinforcements for South Vietnam troubled officials in the Department of Defense’s Systems Analysis Office. After an examination of battlefield statistics, the civilians in Systems Analysis learned that through a combination of in-country recruitment and infiltration down the Ho Chi Minh Trail, the Communists could lose a weekly total of 3,265 men and still fight on indefinitely at present levels. If the U.S. raised the Communist loss rate to 3,696 per week, which was the estimated peak loss rate possible with a U.S. force in South Vietnam of 670,000 troops (the approximate number of troops destined for South Vietnam under Westmoreland’s request), it would still take 10 years to drain the Communist manpower pool. But Systems Analysis determined that the attainment of a peak enemy loss rate of 3,696 per week for ten years in a row was a physical impossibility. U.S. units had not been able to keep enemy losses at peak levels for any sustained period of time in the previous two years. The best they could do was an average. Systems Analysis believed that Westmoreland’s optimal force of 670,000 would in the best-case scenario kill an average of 2,460 enemy per week, a number insufficient to eliminate current and future Communist forces. The DoD statisticians thought even that lower enemy killed-in-action average unlikely because, “The enemy can probably hold his losses (all causes) to about 2000 per week regardless of our force levels or operations.”
One American official familiar with the Communist’s order of battle admitted that what the U.S. was really up against in Vietnam was the Vietnamese birth rate – which at the time was one of the highest in the world. In reality though, the United States was up against more than Vietnam’s birth rate. America’s single greatest problem in Vietnam was that no matter what it did on the field of battle in the South or in the skies above the North, it could not persuade a high enough percentage of Vietnam’s sons and daughters from joining either the Vietcong or the North Vietnamese Army.
On May 4, 1967, Alain Enthoven in Systems Analysis wrote Secretary of Defense Robert S. McNamara, challenging Westmoreland’s assertion that he could shorten the war with additional reinforcements. Enthoven observed that the Communists could sustain their present manpower loss rate indefinitely, regardless of the size of the U.S. troop level in the South. There existed no correlation between higher U.S. troop levels and a higher enemy attrition rate. The enemy, rather than the United States, determined its own casualty rates. Enthoven wrote, “…the size of the force we deploy has little effect on the rate of attrition of enemy forces.” There were two primary reasons the enemy controlled his own losses. First, the enemy initiated 78.7% of the battles. Second, U.S. forces, all too frequently confronted enemy troops in trenches and bunkers. Enthoven stated that his findings were supported by a separate government study that concluded that the enemy initiated 88% of all battles in South Vietnam and that U.S. troops encountered the enemy in trenches and bunkers in 63% of engagements. The Vietcong’s fortified countryside curtailed the effects of U.S. firepower, enabling the enemy to limit his casualties. A key element to effective U.S. attrition was finding and fixing the enemy. Yet, in only 5.4% of engagements did the U.S. tactical commander have reliable information on enemy positions and strength. Furthermore, in only 8.9% of battles did a U.S. unit actually ambush a Communist force. McNamara passed on Enthoven’s conclusions to the president in a May 19, 1967, memorandum, stating, “…new troops would not be able to make a significant difference in the military situation since the enemy controlled the pace of battle. In addition, the North Vietnamese could match any U.S. build-up.”
A few weeks later McNamara wrote the president a sober assessment of the war. He observed, “In South Vietnam, the combat operations have reached a high level of intensity with only slow progress by friendly forces, a situation which is within the power of the enemy to perpetuate….” McNamara recommended that the president reject Westmoreland’s recent request for 195,000 new troops, arguing instead that the U.S. stabilize its force levels and try and buy enough time with the American people to strengthen the South Vietnamese armed forces before the public demanded a U.S. pull out.
Throughout July and early August 1967, the White House, Joint Chiefs of Staff, Secretary McNamara, and General Westmoreland sought to reach a compromise agreement on troop levels for South Vietnam. In the end, McNamara’s June recommendation carried the most weight. Although the president authorized an increase in U.S. troop levels to 525,000 in 1968, Johnson rejected the huge increase to 670,000 troops. Furthermore, the president and McNamara considered the modest increase in troop levels planned for 1968 to be the final troop increment destined for Vietnam.
Johnson’s decision to limit the U.S. troop commitment to South Vietnam marked a crucial turning point in the ground war – continued escalation had been replaced with a policy of stabilization. The decision also signalled an understanding among top policymakers in the White House and at the Pentagon – American military power alone could not save South Vietnam.
 Foreign Relations of the United States, 1964-1968, Volume V, Vietnam, 1967, “Document 149, Notes on Discussions With President Johnson,” (Washington, D.C.: GPO, 2002), 351.
 Foreign Relations of the United States, 1964-1968, Volume V, Vietnam, 1967, “Document 147, Memorandum by the President’s Special Assistant (Komer),” (Washington, D.C.: GPO, 2002), 345.
 Vietnam Task Force, Office of the Secretary of Defense, The Pentagon Papers, Parts IV.C.6 through IV.C.8, “IV.C.6. (b), U.S. Ground Strategy and Force Deployments, 1965-1967, Volume II,” (Nimble Books LLC, 2011), 107-108.
 Foreign Relations of the United States, 1964-1968, Volume V, Vietnam, 1967, “Document 155, Editorial Note,” (Washington, D.C.: GPO, 2002), 367.
 The Senator Gravel Edition, The Pentagon Papers, The Defense Department History of United States Decisionmaking on Vietnam, Volume IV, (Boston: Beacon Press, 1971), 461.
 Vietnam Task Force, Office of the Secretary of Defense, The Pentagon Papers, Parts IV.C.6 through IV.C.8, “IV.C.6. (b), U.S. Ground Strategy and Force Deployments, 1965-1967, Volume II,” (Nimble Books LLC, 2011), 114-115.
 Foreign Relations of the United States, 1964-1968, Volume V, Vietnam, 1967, “Document 177, Draft Memorandum From Secretary of Defense McNamara to President Johnson,” (Washington, D.C.: GPO, 2002), 430.
 Foreign Relations of the United States, 1964-1968, Volume V, Vietnam, 1967, “Document 194, Draft Memorandum From Secretary of Defense McNamara to President Johnson,” (Washington, D.C.: GPO, 2002, 477.