Home / Articles / The U.S.-Philippines Mutual Defense Treaty and Philippine External Defense Forces
On the face of it, the Philippines’ security situation is somewhat puzzling. The country has a mutual defense treaty with the United States, the world’s strongest military power. Yet, China regularly intrudes into what the Philippines claims as its territorial waters in the South China Sea, denies Philippine fishing boats access to those waters (which are well within the Philippines’ exclusive economic zone), and, when the opportunity affords, occupies Philippine-claimed features in the region. In fact, China has stirred up so much uncertainty that international energy companies, normally accustomed to dealing with risk, have refrained from pursuing much-needed offshore exploration projects in Philippines. Meanwhile, Washington has offered its treaty ally what appears to be only modest support.
Certainly, one reason for that situation has been Philippine President Rodrigo Duterte’s “separation” from the United States. For most of his tenure, Duterte has sought to politically distance Manila from its alliance with Washington. But another, less appreciated, reason for that situation has been the dismal state of the Philippine military’s external defense forces. It has been the underlying cause of not only the Philippines’ inability to counter Chinese actions in the South China Sea, but also the ambiguity that had sapped the Philippine-American mutual defense treaty of its deterrent power. Fortunately for Manila, things have begun to change. The Philippines has slowly started to rebuild its external defense forces and won more international recognition for its claims. That, together with a shift in American strategic sentiment towards China, may yet give the treaty a new lease on life.
Path to Dependence
In 1951, the Philippines and the United States signed what would become one of the world’s longest-lived mutual defense treaties, marking its platinum anniversary this year. But Manila never meant for the treaty to be its only bulwark against foreign aggression. Philippine leaders of the era well remembered Japan’s invasion and occupation during World War II. Only 15 years after the war, which had laid waste to large swaths of their country, they built not only one of Asia’s most capable armed forces, but also one with meaningful external defense capabilities. Indeed, the Philippine air force was the first in Asia to acquire jet aircraft, which were flown throughout the Cold War. In 1980, the Philippine air force operated 16 F-5 and 24 F-8 fighters, all reasonably modern for their time. The Philippine navy marshaled two frigates, ten corvettes, and a dozen offshore patrol vessels. Twenty-five years later, in 2005, little of that remained. The air force had no jet aircraft, and the navy could reliably count on only four offshore patrol vessels.
How the Philippines’ external defense forces reached such a nadir can be attributed to a combination of factors: the end of the Cold War, Manila’s need to combat internal insurgencies, and the consistently low budgetary prioritization of its military. However, unlike some other U.S. defense-treaty allies, like Germany, the Philippines did not seek to keep American military forces close at hand (i.e., on its soil) to compensate for its shrinking military capacity. Instead, Manila ejected the United States in a fit of nationalist pique from its bases in 1991. And so, the Philippines—bereft of external defense forces of its own or those of the United States—left itself utterly dependent on the terms of its mutual defense treaty with the United States to deter any potential foreign aggression.
No doubt, having a mutual defense treaty with the world’s strongest military power is useful, but being utterly dependent on it can also be problematic. Even the slightest ambiguity in the treaty’s details could significantly weaken Philippine security. Unfortunately for Manila, the treaty contains several such ambiguous details, which Beijing has exploited to advance its South China Sea ambitions. Hence, Manila has long sought Washington’s clarification of those details. Some are strategic: Precisely what is the United States willing to protect (e.g., the Philippines’ territorial waters, its exclusive economic zone, its larger maritime claims in the South China Sea, or some portion thereof)? Others are tactical: How would the United States, for example, respond if a government-controlled, but nominally non-military entity, such as China’s coast guard or maritime militia, was to take aggressive action against Philippine forces?
Until early this year, the United States had been slow to respond to Philippine appeals. Without a clear-cut mutual defense treaty (or military power of its own), Manila has had few options in dealing with foreign encroachment on what it considers as sovereign territory, something that has frustrated Philippine leaders of all political stripes. It has even led some to characterize the mutual defense treaty between the Philippines and the United States as an “unequal arrangement” and, in Duterte’s case, to try to shelve the treaty altogether.
Of course, clarifying the circumstances under which the United States would come to the aid of the Philippines is no simple matter. Washington must weigh the precedent-setting implications of each case for its security commitments not only to the Philippines, but also to other American allies. Even so, the United States has seemed remarkably unperturbed by the ambiguity in the treaty’s details. As to why, it has probably been because Washington saw some advantage in the ambiguity. From the perspective of American policymakers, the Philippines’ lack of external defense capacity could incentivize Manila to use its mutual defense treaty with the United States—its only form of real power—to assert its South China Sea claims. In short, Washington fears that Manila might take some provocative action that—whether inadvertently or not—brings the United States into a direct conflict with another country, most worryingly China.
Revival of Philippine military power. The first stirrings of change in the strategic environment came in 2010. It was then that former Philippine President Benigno Aquino III began the long process to resurrect his country’s external defense forces. He shepherded through the Philippine Congress key legislation that rationalized and reformed military procurement. His administration then architected a 15-year modernization program for the Philippine military. And, most importantly, he secured legislative funding for it. Although since then parts of the program have been delayed, his successor, Duterte, has continued it. In 2015, the Philippine air force received its first jet-powered combat aircraft in over 40 years, and, in 2020, its navy commissioned its very first missile-capable warships. Better still for Manila’s South China Sea claims, the Philippine Marine Corps is slated to receive its first battery of Indian-made, ground-launched BrahMos anti-ship cruise missiles, whose range covers most of the Philippines’ 200-nautical-mile exclusive economic zone.
While the Philippines’ new military assets may still be too weak to give China much cause for serious concern, they do signal to the United States a real commitment by Manila in its own external defense. They also help alter the Philippines’ security relationship with the United States. Without Manila’s complete dependence on the mutual defense treaty, American policymakers can no longer rely on the treaty’s ambiguities to act as either a restraint on or a lever over Philippine actions. Indeed, as Philippine external defense forces become stronger, they provide greater incentive for Washington to clarify the treaty’s ambiguities.
International recognition of Philippine claims. The Philippines also strengthened its hand over its mutual defense treaty with the United States in another way. In 2016, the Aquino administration won a three-year legal case against China’s “nine-dash line” claim in the South China Sea at the Permanent Court of Arbitration (PCA). While much of the attention that the PCA ruling has received has been focused on its invalidation of the Chinese claim, it also gave Philippine maritime claims a boost. The court affirmed the Philippines’ sovereign rights within its exclusive economic zone and found Philippine fishing activities around Scarborough Shoal to be lawful. Such international recognition has given Manila added leverage in its treaty relationship with Washington. The more international recognition the Philippines can garner for its claims, the harder it becomes for the United States not to include them under the treaty.
Change in American strategic sentiment. Meanwhile, how America sees its relationship with China also changed. For much of the last quarter century, the United States—not to mention most of Asia—hoped that globalization would turn China into a “responsible stakeholder” and transform Chinese attitudes on many issues, including sovereignty disputes in the South China Sea. It did not. That recognition, together with the realization that China had become a competitive danger to the United States, shifted American strategic sentiment towards China by the late 2010s. During the presidency of Donald Trump, Washington became decidedly less concerned with tiptoeing around Beijing’s self-declared “core interests” (those which it is willing to fight), than it once was.
In Trump’s last full year as president, the United States not only openly challenged China’s expansive claims in the South China Sea, but also sold Taiwan $5 billion worth of advanced weaponry. So far, President Joseph Biden has continued his predecessor’s harder line against China. Early on, his administration clarified that the Philippine-American mutual defense treaty would indeed obligate the United States to aid Philippine forces if China’s maritime militia were to attack them. In July 2021, Washington reiterated that treaty commitment as well as the Trump administration’s policy that deemed nearly all of Beijing’s maritime claims in the South China Sea to be illegitimate. All of which suggests that the Philippines can expect an easier time securing American support in the years ahead.
Turning a Corner?
In the late 2000s, the Philippines’ decades-long under-resourcing of its military had left the country utterly dependent on its mutual defense treaty with the United States for external defense. That situation not only allowed Beijing to advance its South China Sea interests by exploiting the treaty’s ambiguities, but also limited the ability of Philippine leaders to devise effective strategies to counter Chinese assertiveness in the region. The slow but tangible revival of the Philippines’ external defense forces, along with the greater international recognition of Philippine claims and the shift in American strategic sentiment towards China, has created the opportunity to reduce the incentives for ambiguity in the treaty and, in doing so, restore its deterrent value.
Considering China’s provocative use of its maritime militia in the South China Sea (particularly near Whitsun Reef) in early 2021, restored deterrence could not come soon enough for the Philippines. Indeed, in a sign of Manila’s exasperation with Chinese behavior, Duterte reversed his earlier effort to terminate the Visiting Forces Agreement, another security arrangement between Manila and Washington, in July 2021. If Duterte has truly shifted from his policy of accommodation to one of limited balancing with China, as some say, a clearer mutual defense treaty would surely come in handy. For ultimately that is more likely to benefit the long-term security of the Philippines than any gamble on Chinese benevolence.
The views expressed in this article are those of the author alone and do not necessarily reflect the position of the Foreign Policy Research Institute, a non-partisan organization that seeks to publish well-argued, policy-oriented articles on American foreign policy and national security priorities.
 Institute for International Security Studies, The Military Balance 1979-1980 (London: Arms and Armour Press, 1980), p. 74.
 Renato Cruz De Castro, “From Appeasement to Soft Balancing: The Duterte Administration’s Shifting Policy on the South China Sea Imbroglio,” Asian Affairs: An American Review, Sep. 10, 2020.