• Renato Cruz De Castro

Whitsun Reef Stand-Off: A Test Case for the Applicability of the 1951 PH-US MDT in the 21st Century?

Updated: Sep 13, 2021

Photo Source: Wikipedia/ABS-CBN

The Philippines is the oldest United States’ (U.S.) treaty ally in Asia as the two countries signed what would become one of the world’s longest active mutual defense treaties, the Philippines-U.S. Mutual Defense Treaty of 1951 (MDT). During the Cold War, the Philippines hosted major U.S. naval and air bases until 1992. During the early 21st century, the Philippines became an important partner in the American global war on terrorism. The country allowed the temporary deployment of U.S. Special Forces on the southern Philippine-island of Basilan to assist the Armed Forces of the Philippines (AFP) in its counter-terrorism operation against the dreaded terrorist group, the Abu-Sayyaf. In the second decade of the 21st century, the Philippines facilitated the Obama Administration’s strategic rebalancing to Asia by signing the 2014 Enhanced Defense Cooperation Agreement (EDCA) with the U.S. in April 2014.

Twenty-first century Philippine-U.S. alliance, however, is hounded by ambiguity of its sole legal foundation, the 1951 MDT. The treaty does not explicitly commit each ally to assist each other in the event of an armed attack as it simply established a consultative security relationship which is expressed in broad and ambiguous terms. Article Four of the MDT states that “Each Party recognizes an armed attack in the Pacific Area on either of the Parties would be dangerous to its own peace and safety and declares that it would act to meet the common dangers in accordance with its constitutional process.” The Treaty does not explicitly commit the U.S. to come to the Philippines’ assistance in maritime areas that are disputed by the Philippines and other states as it leaves room for different interpretations.

The 1951 MDT does not provide for any preparations and planning to address hypothetical cases of armed aggression against any of the parties. This is similar to other U.S. mutual defense treaties with its other Asian and Pacific allies. This was because the Department of Defense (DOD) opposed the inclusion of any provisions involving to military planning and joint planning to all U.S. mutual defense treaties signed with Australia and New Zealand under the Australia-New Zealand-United States Treaty (ANZUS) Treaty, and separately with the Republic of China (Taiwan), Japan, and later, the Republic of Korea. The treaty’s ambiguous language was designed so as not to supersede the Military Bases Agreement (MBA) of 1947 and was intended to reassure the Philippines against a former common enemy (but a forthcoming American ally) against communist China in the early 1950s, Japan. The ambiguity also served Washington’s tacit agenda when the MDT was being formulated and negotiated with Manila in the early 1950s – to commit the Philippines to providing military assistance to American occupation forces in occupied Japan. Consequently, most U.S. interpretations of the MDT have fallen short of a clear-cut security guarantee or have not specified what type of assistance might be offered or even if it will be offered at all to the Philippines. The MDT’s ambiguity would haunt the alliance way into the 21st century.

Making the MDT relevant in the 21st Century

In the second decade of the 21st century, U.S. policy remained vague and ambiguous regarding its treaty commitment to the Philippines with regards to the South China Sea dispute. Washington stops short of any reference to an automatic response if an armed conflict erupts in the South China Sea. Instead, it emphasized the treaty’s diplomatic, rather than the military, deterrence. The State Department’s official position states that since the U.S. is a treaty ally of the Philippines, China cannot simply assert that events in the South China Sea, including the contested islands, are not any of Washington’s business.

However, recent U.S. attitude towards China has become more critical in the light of the U.S.-China strategic competition. Consequently, there have been obvious changes in Washington’s thinking towards the vulnerabilities felt by Manila in the face of Beijing’s coercive behavior in the South China Sea. This change became apparent when Philippine Defense Secretary Delfin Lorenzana announced that the Department of National Defense (DND) is reviewing the 1951 MDT and its relevance in the 21st century in December 2018. He declared that the time has arrived for the MDT “to be revisited, given that its provisions were formulated in the early 1950s.” In March 2019, during his official visit to the Philippines, then Secretary of State Michael Pompeo addressed Secretary Lorenzana’s concern about the MDT when declared: “As the South China Sea is part of the Pacific; any armed attack on Philippine forces, aircraft or public vessels in the South China Sea will trigger mutual defense obligations under Article 4 of our mutual defense treaty.” This statement would be tested two years later in Whitsun Reef.

A Test Case for the MDT?

On March 20, 2021, Secretary Lorenzana officially informed the Filipino nation about the presence of around 220-blue hulled Chinese fishing vessels moored in line formation at Julian Felipe Reef (international name Whitsun Reef). He issued a statement announcing that the Philippines is ready “to defend its national sovereignty and protect the country’s marine resources.” The following day, Department of Foreign Affairs (DFA) Secretary Teodoro Locsin filed a diplomatic protest with the Chinese Embassy in Manila.

The U.S. promptly announced its support to the Philippines as U.S. National Security Adviser Jake Sullivan called his Filipino counter, Hermogenes Esperon, to emphasize Washington’s backing to the Philippines and the applicability of the MDT to the area. On April 9, U.S. Secretary of State Antony J. Blinken called Secretary Locsin to express Washington’s concern over the massing of Chinese maritime militia vessels in the South China Sea, and more importantly, to reaffirm the applicability of the 1951 MDT in the South China Sea. U.S. Department of Defense Secretary Llyod Austin called Secretary Lorenza to convey to his Philippine counterpart that the U.S.S.-Theodore Roosevelt, its escorts, along with the amphibious assault ship the U.S.S. Makin Island, were on their way the South China Sea.

The DFA released a statement emphasizing that the MDT has “stood strong for nearly 70 years, in the light of the recent geopolitical developments and challenges in the Asia Pacific region, particularly in the West Philippine Sea.” The DND then raised the prospect of seeking assistance from the U.S. and its security partners as a department spokesperson announced: “As the situation in the West Philippines Sea evolves, we keep all our option open in managing the situation, including leveraging our partnerships with other nations such as the United States.”

President Duterte, through a statement, however, rejected the U.S. offer of assistance. Instead, he made a scything revoke against the U.S. as he expressed that it is doubtful on whether or not the Philippines could count on its ally in case of a full-blown conflict in the West Philippine Sea. Nevertheless, the incident showed that top Biden Administration defense and foreign affairs officials worked effectively with their Filipino counterparts to counter China’s coercive attempt to control Whitsun Reef and to reaffirm the MDT’s applicability in the South China Sea imbroglio with America’s longest treaty ally.


Renato Cruz De Castro is a distinguished university professor in the International Studies Department, De La Salle University, Manila, and holds the Dr. Aurelio Calderon Chair in Philippines-American Relation. He also writes opinion columns for two Philippine newspapers. He has published more than 100 articles and book chapters on Foreign Policy Analysis, International Relations, and Security Studies that have been published in the Philippines and in fourteen other countries. In 2019, the Board of Trustee of the Philippine Political Science Association (PPSA) announced that Professor De Castro is the most prolific Political Scientist in terms of scholarly publication, and one of the top 10 Filipino Political Scientists in terms of number of citations.

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