With NATO labeling China a “systemic” threat to the international order and the Pentagon debating the creation of a permanent naval task force in the Pacific, the prospect of war with China has never felt more concrete. Yet if Washington and Beijing ultimately go to war, it might have less to do with today’s great power competition than with how Donald H. Rumsfeld and Cyrus Vance dealt with the American relationship with the Philippines in the wake of the Vietnam War.

Concerned by American inaction and unwillingness to provide emergency military assistance as Saigon fell in 1975, Philippine President Ferdinand Marcos undertook a review of his county’s alliance with its former colonial ruler, and the continued American military presence in the Philippines. He demanded formal statements clarifying U.S. obligations under the 1951 Mutual Defense Treaty (MDT), including how the treaty applied in the pivotal South China Sea. The resulting formulation has underpinned U.S. policy for more than 40 years and leaves the United States committed to a position that could bring it into conflict with China over this crucial waterway.

The South China Sea emerged as a geopolitical flash point in the 1970s amid a surge in offshore oil exploration by nearby countries including the Philippines. Yet the dispute was also intrinsically shaped by the waning days of the Vietnam War. As South Vietnam floundered, both North Vietnam and the People’s Republic of China jockeyed for maritime features to bolster their territorial claims in the disputed sea.

This scramble had a spillover effect on the U.S.-Philippines alliance. The fall of Saigon had triggered an existential crisis in Manila. Since gaining independence in 1946, the Philippines had continued to depend upon the United States as the guarantor of its national security.

Yet Washington’s handling of the Indochina conflict brought the reliability of this arrangement into question. U.S. diplomats in Manila related widespread dismay over the “abandonment of Cambodia and Viet-Nam” and calls for the Philippines to end its reliance on Washington. Marcos demanded a renegotiation of the Military Base Agreement (MBA) governing the U.S. military installations in the Philippines and a clarification of the MDT. The potential loss of or restriction upon the U.S. bases was particularly distressing for Washington. They were among the largest American military bases in the world and remained vital to defending U.S. interests in Asia.

The South China Sea increasingly became central to these negotiations. North Vietnam’s April 1975 takeover of Southwest Cay in the Spratly Islands spurred Marcos to have a pointed discussion with U.S. Ambassador William Sullivan over whether and how the MDT applied to these islands.

This conversation led to a State Department review, which determined that the MDT would not, in fact, cover an attack by a foreign aggressor on Philippine forces stationed in the Spratly Islands. Yet American diplomats tried to finesse the issue, with Sullivan warning that if this judgment “were ever to come to [the] attention of Filipinos,” it “would confirm their worst fears and suspicions concerning [the] value of U.S. treaty commitments.”

Marcos, however, rejected vague answers. During a visit by Deputy Secretary of State Charles Robinson in August 1976, he pressed Robinson for a firm statement that the U.S. was committed to protecting those engaged in Philippine oil exploration near Reed Bank, a maritime feature near the Spratly Islands. Dissatisfied by Robinson’s answers, Marcos formally requested an explicit statement on American MDT obligations in this area.

Marcos made clear that an unsatisfactory answer would have an impact on the military base negotiations — his ultimate trump card.

With ambiguity and evasions having failed, Washington needed to craft an answer that would meet Philippine expectations without prematurely committing the United States to a future conflict in the South China Sea. Although Washington regarded Philippine concerns as genuine, it also feared provocative actions by Manila that might embroil the United States in a conflict that it neither sought nor supported at a time when the American public wanted little to do with another conflict in Southeast Asia.

Rumsfeld, then the defense secretary, came up with a solution. As he explained to President Gerald Ford, the key was tying the U.S. commitment to defend the Philippines to “what the Philippines are doing rather than where in the disputed area they may be doing it.” The United States could promise to protect the Philippines’ “armed forces, public vessels and aircraft” in the South China Sea as long as they weren’t behaving provocatively. This gave the United States some flexibility, while also reassuring Marcos that the United States wouldn’t renege on its commitment because of location.

Secretary of State Henry Kissinger was supposed to present this revised formula to the Philippine secretary of foreign affairs, Carlos Romulo, on Oct. 6, 1976. Instead, Kissinger swerved. He bluntly explained that the United States was fully prepared to defend “the metropolitan area of the Philippines.” But if Marcos’s government wanted “the Reed Bank and the Spratlys included,” then the United States would “insert a waffling clause” — which would create new problems and blurriness.

Romulo chose the first option, asserting that the Philippines wanted to exclude “controversial areas,” and only cover the metropolitan territory of the Philippines.

The result was an official document reaffirming American obligations to respond to an attack on the Philippines and stating that the United States was “pleased to receive the assurances of the Philippine government that it has no intention of involving the United States in the resolution of disputed territorial claims.” This language avoided potential U.S. involvement in a war over control of the South China Sea. This note seemingly diffused Philippine concerns that the United States would abandon its commitments, but negotiations over the Military Base Agreement collapsed following Ford’s 1976 election loss.

The new Carter administration resumed talks, and the sides reached an agreement in late 1978. However, days before the amended MBA was to be signed, Marcos revived the defense treaty issue, insisting that the Kissinger note from 1976 was now insufficient.

In response, Vance, now secretary of state, sent Romulo a diplomatic note outlining America’s interpretation of the MDT. The note bluntly stated that an attack on Philippine forces or public vessels “would not have to occur within the metropolitan territory of the Philippines or island territories” for the treaty to be invoked.

Vance’s letter reflected the Rumsfeld formulation and established that while the United States did not take a stand on who owns the South China Sea, Philippine forces there were nevertheless protected by the MDT. Successive American administrations reaffirmed this framework and it remains U.S. policy today.

Moreover, this understanding of the MDT has shaped events in the South China Sea. Notably, in 1999 the Philippines intentionally grounded a naval vessel, the BRP Sierra Madre, on Second Thomas Shoal in the Spratly Islands to prevent China from seizing the shoals. The Sierra Madre’s rusting hulk may not dissuade aggression from China, but the knowledge that any attempt to forcibly dislodge the Philippine vessel could risk conflict with the United States has proved a potent deterrent. Kissinger probably would have approved of the ploy. As he told Romulo in 1976, “if you seize the territory, it’s always easier to handle.”

The Philippines has not seized more territory, but rather sought to resolve the dispute through diplomatic and legal means. Yet rising Chinese assertiveness and harassment of Philippine vessels has kept the South China Sea simmering. Today, with the Philippines increasing its maritime activities in response, the Rumsfeld-Vance framework ensures that the United States cannot remain aloof. Ultimately, if tensions in the South China Sea erupt into violence, the formula adopted in the 1970s means Washington will either have to abandon a longtime ally or embrace a conflict with China that it otherwise may have wished to avoid.