Was It Legal for the U.S. to Bomb Cambodia?

Is it acceptable to engage an enemy on the territory of a third country? It’s a question that has confronted Washington policymakers in recent years over ISIS forces in Syria and Qaeda operatives in Afghanistan and Pakistan. But in 1967, it was asked most urgently over the use of Cambodian territory by Vietnamese communists. What Washington policymakers decided then, and the way they continued to address the question over the following years, has had a significant impact on the shape of American foreign policy ever since.

The National Liberation Front, also known as the Viet Cong, and North Vietnam established camps and supply routes on the Cambodian side of the border with South Vietnam to resupply their forces and undertake military operations against the Saigon government and its allies. As an avowedly neutral party in the war, the Cambodian government did not formally approve of the Vietnamese communists using its territory as a base area, nor would it give permission to American or allied forces to enter its jurisdiction to confront their enemies.

This left decision makers in Washington in a bind. On the one hand, respecting Cambodian sovereignty allowed the enemy a sanctuary or safe haven and so hindered the American war effort. On the other, taking action against the enemy on Cambodian soil risked a public backlash, both within the United States and around the world.

In December 1967, this dilemma came into sharp focus for President Lyndon Johnson and his advisers when Gen. William Westmoreland, the commander of American forces in Vietnam, recommended an attack on enemy units resting and regrouping in Cambodia after the Battle of Dak To the previous month.

Westmoreland saw an easy target — intelligence showed no evidence that the enemy units were digging in and constructing bunkers — and on Dec. 5 he sought approval for three days of airstrikes by B-52 bombers. Westmoreland acknowledged that B-52 strikes would leave “a clear signature in Cambodian territory,” meaning news of the operation would be hard to keep under wraps, but he was adamant that possible bad publicity was worth the risk. “I feel strongly that the opportunity which exists is sufficiently important that it should be exploited now,” he wrote to Gen. Earle Wheeler, the chairman of the Joint Chiefs of Staff, “and if necessary explain our actions as hot pursuit by fire in an uninhabited area.”

A recent spike in news media attention might have helped Westmoreland’s cause. Two weeks earlier, journalists from The Associated Press, tipped off by Westmoreland’s own staff, had reported the discovery of an enemy camp four miles inside Cambodia. And in a late November television interview, former President Dwight Eisenhower criticized “that sort of sacrosanct idea of a line on the ground that no one can see, but it’s on your maps, and you must respect it.”

Publicly supporting the sort of action in “hot pursuit” that Westmoreland and the Joint Chiefs would pitch in private a week later, Eisenhower was adamant that “if you’re chasing some people and they just step over into Cambodia or Laos, I wouldn’t — it wouldn’t bother me. I’d go at ’em as long as they’d come in there in the first place.”

Johnson discussed Westmoreland’s request in two meetings with his closest advisers on the war on Dec. 5. Wheeler pressed Westmoreland’s case: “We only wish Cambodia would be neutral — honest to God neutral, too. Anyone else would not permit enemy troops to use their territory for sanctuaries.” But he was in a clear minority. Secretary of Defense Robert McNamara, who had announced his pending resignation the previous week, spoke up against granting Westmoreland’s request, telling the group that he was “scared to death of a policy based on an assumption that by going somewhere else we can win the war.”

His State Department counterpart, Dean Rusk, was similarly wary, speculating that Westmoreland’s proposed action could “change the entire character of the war.” Johnson put Westmoreland’s request on hold.

Westmoreland’s proposal to explain his recommended action in Cambodia as “hot pursuit by fire” was an attempt to solve the crux of the Cambodian dilemma as it appeared to American policymakers throughout the Vietnam War: how to warp the red line that a sovereign border represented without ignoring it entirely. As the economist Thomas Schelling put it the previous year in “Arms and Influence” — something of a strategic playbook for key architects of the American war in Vietnam — the “purpose of invoking ‘hot pursuit’ is not merely to find an excuse for it but to identify a limitation in intent, to let the enemy appreciate that his is not an abandonment altogether of some previous restriction but an allowable departure under the rules of the game.”

A number of Johnson’s advisers were skeptical of this logic. They thought that rather than signaling limited intent, an appeal to a right of hot pursuit would more likely be seen as feeble cover for a significant change in policy.. Averell Harriman, ambassador at large and one of Johnson’s foreign policy “wise men,” tried to convince the president that “the world at large (and a substantial section of the American public) has not yet been presented with convincing evidence that expanding the war into ‘a tiny, helpless country’ is justified.”

In any case, despite the assumption of Westmoreland and Schelling, hot pursuit was not widely accepted as an allowable departure under the rules of the game. It had no standing in international law (except within the law of the sea, which allows for the hot pursuit of vessels into international waters), and since at least March 1965 the State Department Office of the Legal Adviser had rejected it as an appropriate justification for crossing the Cambodian border. The lawyers insisted instead that any legal argument for border crossing would need to rest on the right of self-defense guaranteed by Article 51 of the United Nations Charter.

The legal basis for the American bombing of North Vietnam rested on the collective self-defense of South Vietnam. But the logic of self-defense regarding Cambodia was different: Whereas the government of North Vietnam could conceivably be linked to military attacks against the government of South Vietnam, the government of Cambodia could not. Absent a sudden threat that left no time for deliberation, Washington would therefore need to show Cambodian “connivance” in the use of its territory before launching any attack on Cambodia. The Cambodian head of state, Prince Norodom Sihanouk, often appeared to be more friendly to the communists than to the Americans, but substantial evidence of connivance did not surface.

Particular military units engaged with the enemy had authority to fire over or cross the border if necessary to defend themselves. (On Dec. 5, Rusk suggested that this authority might still allow Westmoreland some freedom of action: “I would have thought that Westy would have drug his shirttail along the Cambodian border and drawn enemy fire. Then the rules would permit him to shoot back across the border when fired upon.”) But beyond this, and the limited covert reconnaissance operations code-named Daniel Boone, no further authority was granted. On Dec. 18, Johnson effectively ended the debate over Westmoreland’s request. The United States would instead focus on trying to convince the Cambodian government to be more assertive in the defense of Cambodian neutrality, or to let others act to preserve that neutrality.

Johnson’s successor in the White House made a different decision. President Richard Nixon approved B-52 strikes on suspected enemy locations in Cambodia from March 1969, and at the end of April 1970 he sent American ground forces across the frontier to find and destroy enemy sanctuaries. The catalyst for the 1970 Cambodian incursion (or invasion) was the overthrow of Sihanouk and the establishment of a more pro-American government in Phnom Penh. But even this government sought to preserve Cambodia’s neutrality, meaning it could not formally invite American troops into the country.

Nearly a decade after the incursion, Nixon’s national security adviser, Henry Kissinger, wrote that the “moral significance” of the distinction between the Johnson and Nixon policies regarding Cambodia escaped him, even if the distinction seemed self-evident to the many people and foreign governments that objected to and protested the action.

The legal significance of Nixon’s action in Cambodia was more subtle, yet not inconsequential. A post-facto legal justification for the incursion was slow in coming, but on May 28, Nixon’s State Department legal adviser laid out a novel extension to the right of self-defense, arguing that the incursion was justified because the Cambodian government could not or would not defend its neutral status and prevent Vietnamese communists from using its territory.

Across a half-decade of earlier legal advice concerning Cambodia’s role in the Vietnam War, lawyers in the Johnson administration do not appear to have acknowledged the existence of such a standard, now known as the “unwilling or unable” doctrine. But since its articulation in 1970, the “unwilling or unable” doctrine has played an increasingly important role in justifying American interventions abroad. How American leaders grappled with the Cambodian problem during the Vietnam War has important implications for the use of military force by the United States today.