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 recently turned an alarming, bright red, signifying Communist domination. In the midst of the sea of red were the towns and cities still under Allied control – marked by small, white spots. 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 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. There were several reasons these men had come to this conclusion. 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 to the nation, then the U.S. needed to strengthen its military position in South Vietnam; otherwise, the Communists would be in a position to 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 – something that was not possible with the Communists occupying large parts 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. He stated, “…We must win the race to the countryside, go on the offensive, re-establish 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 military bases. 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. forces from South Vietnam required that the U.S. literally shove Vietcong and North Vietnamese units away from its military positions. Any attempted withdrawal to the coast, with the Communists close-at-hand, 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 wake of the Tet Offensive, 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 before, 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 1968. In Communist hands, that population would provide the Vietcong with the military manpower to undermine South Vietnam’s long-term political cohesion and military security. Additionally, 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. Finally, no country could survive long if its urban centers were cut off from their hinterlands. Cities and towns need the surrounding countryside for produce, markets for finished products, and cheap labour. A nation-state, governing only isolated enclaves, would eventually succumb, as Maoist guerrilla theory prophesied, to the rural enemy, who would tighten a 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.
Vietnam experts at the White House, Pentagon, and Foggy Bottom recognized that GVN officials and South Vietnamese military commanders would head for the exits as soon as it dawned on them 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 without the domestic and international perception that it had suffered a humiliating defeat, it had to retake sizeable areas of rural South Vietnam and it had to do so in a hurry.
Reoccupying the rural areas surrounding the Allied enclaves became the geographical equivalent of the “decent interval.” In this case, 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 the inevitable 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.
The U.S.’s need for a geographical decent interval had a dramatic influence on U.S. military operations in the aftermath of the Tet Offensive. In March 1968, the United States began a counter-offensive across rural South Vietnam, which relied heavily upon airstrikes. The counter-offensive 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 Vietnamese launched a major offensive across the DMZ in near future; and 3) to give the GVN and ARVN a little more time to survive a large-scale Communist offensive across the DMZ after the U.S. pulled out of the country.
The effort to establish a large geographical decent interval continued during the Nixon administration. 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 base areas and Allied units in I Corps. In 1970, President Nixon ordered U.S. and South Vietnamese mechanized units into Cambodia. This invasion attempted to push the Communists away from the South Vietnamese-Cambodian border. In 1971, Operation Lam Son 719 sought to eject the Communists from their base areas around Tchepone, Laos. Lam Son 719 not only failed in its primary objective, it ended with the rout of the South Vietnamese Army.
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.