On Allied military maps, South Vietnam at the end of the Tet Offensive looked like a country suffering from some sort of an infectious disease. Large segments of the countryside had turned an alarming, bright red, signifying Communist domination, while small, white dots denoted the urban enclaves still under Allied control. These maps indicated that the geographical reach of the Vietcong and North Vietnamese had become so extensive, and Allied territorial control so insignificant, that the “disease” of Communism might soon overwhelm South Vietnam.
President Johnson, and his close circle of military advisors, who included Secretary of Defense Clark Clifford, National Security Advisor Walt W. Rostow, Chairman of the Joint Chiefs of Staff General Earle Wheeler, and General William C. Westmoreland, understood that the United States and South Vietnam had to quickly retake the areas lost to the Vietcong and North Vietnamese during the Tet Offensive. These men had come to this conclusion for several reasons. First, and most importantly, the longer the Communists held their new territories, the harder it would be to dislodge them. Given enough time, the guerrillas and their civilian supporters would fortify their hamlets and the surrounding agricultural landscape, guaranteeing that any later effort to push them out would be costly in Allied casualties. Secondly, if the Vietcong and North Vietnamese consolidated their military and political hold over their new territories, they would have access to a larger manpower pool and tax base, which would enable them to rapidly replace their Tet casualties and mount additional offensives in the months and years ahead. Finally, if the United States and North Vietnam engaged in negotiations over a cease-fire on the ground in South Vietnam, which looked increasingly likely after Johnson’s March 31st televised address, then the U.S. needed to strengthen its military position in South Vietnam; otherwise, the Communists would likely demand political and military concessions from the United States – concessions the President might be forced by domestic considerations to accept. Johnson wanted to negotiate from a position of strength – which wouldn’t be possible if the Communists continued to hold so much of South Vietnam.
In early 1968, Secretary of Defense Clark Clifford advised Johnson that the Allies should return to the countryside quickly in the wake of Tet or risk serious long-term erosion of influence there. “…We must win the race to the countryside, go on the offensive, reestablish security in the rural areas, and restore the government’s presence in the villages.” At the same time that he argued for a rapid return to the countryside, Clifford tried to convince anyone who would listen to him within the administration that the U.S. should begin to disengage from the war.
Clifford’s advocacy of an Allied offensive across rural South Vietnam in conjunction with a program of military disengagement were not contradictory proposals. The Secretary of Defense understood that a U.S. exit from South Vietnam would be far more difficult if the Communists held the countryside and their military units remained in forward positions near South Vietnam’s towns, cities, and coastal enclaves. Under those circumstances, an American withdrawal would look as though the U.S. had been pushed out of South Vietnam. And according to Clifford, that perception, if widely held by America’s friends and enemies, would seriously diminish the U.S.’s global standing. Furthermore, the safe extraction of U.S. troops from South Vietnam required that American forces first shove Vietcong and North Vietnamese units away from their base camps. Any attempted withdrawal to the coast, with the Communists close behind, risked high U.S. casualties.
The prospect of a cease-fire sometime in 1968 also influenced Clifford’s thinking. A cease-fire followed by a unilateral U.S. pull-out, which some in the media and in Congress urged on the President in the spring of 1968, entailed grave dangers for the South Vietnamese government (GVN). If the Communists controlled most of rural South Vietnam at the time a cease-fire went into effect, the South Vietnamese military and government would likely collapse – either just prior to, or during, a U.S. pull-out. The reasons for this dire assessment had to do with demographics and economics. Rural South Vietnam contained the bulk of the South’s population. In Communist hands, that population would provide the Vietcong with the military manpower to undermine South Vietnam’s long-term social and political cohesion and military security. Furthermore, South Vietnam possessed a predominantly agricultural economy. Thus, without the countryside, the Saigon regime could never hope to achieve economic self-sufficiency, let alone acquire the capital for industrial development. South Vietnam would forever be poor, dependent, and economically vulnerable. Additionally, no country could survive long if its urban centers were cut off from their hinterlands. Cities and towns must have access to the surrounding countryside for produce, markets for finished products, and cheap labour for infrastructure projects. A nation-state, governing only isolated enclaves, would eventually succumb, as Maoist guerrilla theory prophesied, to the rural enemy, who would tighten its noose around the towns and cities until they fell under the sway of the Communists. If the Communists dominated the countryside at the time of a cease-fire, South Vietnam was doomed.
GVN officials and South Vietnamese military commanders would head for the exits as soon as they recognized that without the countryside – and its resources and population – South Vietnam had no chance of survival after a U.S. withdrawal. Joint Chiefs of Staff Chairman General Wheeler expressed strong opposition to a cease-fire for this very reason. In a March 20, 1968, memorandum to Clifford, he wrote, “…there are at present substantial areas of the country occupied by VC/NVA forces. If a cease fire were offered and accepted, these areas would remain under control of the enemy. He could resupply these forces and augment them with impunity in order to expand his control over the countryside and/or prepare for renewed offensive operations at a time and at places of his choosing. Moreover, in such a situation, the South Vietnamese Armed Forces would probably lose all offensive spirit and could not be relied upon to withstand renewed widespread attacks…A proposal for a cease fire, if accepted by the enemy, would place Allied forces in a position of maximum military disadvantage.”
In order for the U.S. to get out of South Vietnam without the rapid disintegration of South Vietnam, and to forestall the domestic and international perception that it had suffered a humiliating defeat, the U.S. had to retake sizeable areas of rural South Vietnam. Reoccupying the rural areas surrounding the Allied enclaves became the geographical equivalent of the “decent interval.” In this cases, the interval was the actual territory, and the land distance, separating Allied and Communist forces. The greater the physical distance between the two opposing forces and between the insurgents and the Allied enclaves, the more time there would be between a U.S. withdrawal and a South Vietnamese collapse. That extra time would allow the U.S. to place the blame for South Vietnam’s fall on the South Vietnamese themselves, making it harder to blame the U.S. for the loss of the country.
The need to establish a geographical decent interval significantly influenced U.S. military operations in the months and years after Tet. In March 1968, the United States began a counter-offensive across rural South Vietnam, which relied heavily upon airstrikes, that drove the Communists away from their forward positions near South Vietnam’s towns and cities. Then, on March 31st, the Johnson administration directed that U.S. air strikes against North Vietnam be confined to the area south of the 20th Parallel. The purpose of this new, restricted air campaign was threefold: 1) to prevent the concentration of Communist troops and supplies close to South Vietnam’s northern border; 2) to provide Allied forces both time and space in case the North launched a major offensive across the DMZ; and 3) to give South Vietnam a little more time if and when the U.S. withdrew. In 1969, U.S. units attempted to push the Communists out of the A Shau Valley west of Hue. This operation sought to increase the territorial distance between Communist forces and Allied forces in I Corps. In 1970, the U.S. attempted to establish a wider geographical interval between South Vietnam and the territory controlled by the Communists in Cambodia. In 1971, South Vietnamese units, supported by U.S. air power, tried to eject the Communists from their base areas around Tchepone, Laos. Lam Son 719 not only failed to achieve its objectives, it ended with the humiliating retreat of the South Vietnamese from Laos.
During General Creighton Abrams’s tenure as MACV chief, U.S. ground forces focused their tactical operations on the Communist logistical system, or what Abrams referred to simply as “The System.” Since Communist forces placed their foodstuffs, supplies, and ammunition out in front of their units before an operation, rolling up “The System” established a geographical decent interval in areas where it succeeded. The U.S. policy of Area Control did the same. By destroying rural villages and hamlets and forcing millions of peasants to relocate to South Vietnam’s roadways and urban slums, the United States created a no-man’s land, or interval, between Allied positions and Vietcong and North Vietnamese base areas.
Historians often think of the Decent Interval as having originated with President Nixon and as having been a policy focused on time – that is – the time between a U.S. withdrawal from South Vietnam and the collapse of the Saigon regime. Yet, the original Decent Interval, first formulated and implemented by President Johnson, and later adopted by Nixon, emphasized the importance of space; or more specifically, the geographical interval between Allied territory and Communist territory. The geographical interval, and the desire of two presidents to widen it as much as possible, had an overriding influence on U.S. military policy in Vietnam from early 1968 until the Paris Peace Accords of 1973.
 Vietnam Task Force, Office of the Secretary of Defense, The Pentagon Papers, Parts IV.C.6 through IV.C.8, “IV.C.6. (c), U.S. Ground Strategy and Force Deployments, 1965-1967, Volume III,” (Nimble Books LLC, 2011), 55.
 Foreign Relations of United States, 1964-1968, Volume VI, Vietnam, January-August 1968, “Document 144, Memorandum From the Chairman of the Joint Chiefs of Staff (Wheeler) to Secretary of Defense Clifford, March 20, 1968,” (Washington, D.C.: GPO, 2002), 423.
 Foreign Relations of United States, 1964-1968, Volume VI, Vietnam, January-August 1968, “Document 169, Editorial Note,” (Washington, D.C.: GPO, 2002), 494.