The U.S.-Philippines alliance took a critical hit this week. In what appears to be a retaliation for a canceled U.S. visa for one of his personal allies, Philippine President Rodrigo Duterte formally notified the U.S. Embassy in Manila of his intent to end the Philippines-U.S. Visiting Forces Agreement (VFA).

What is this agreement and why is this so important to the United States? Here’s what you need to know.

What is the VFA?

The U.S. military operates around the world thanks to Status of Forces Agreements (SOFA) in 100 or so countries. Similarly, the VFA spells out the rules, guidelines and legal status of the U.S. military when operating in the Philippines. The VFA also affirms the 1951 Mutual Defense Treaty as well as the 2014 Enhanced Defense Cooperation Agreement — agreements that enable the U.S. military to conduct joint exercises and operations in the Philippines.

Terminating the VFA would leave the U.S. military without any legal or operational standing in the Philippines — and that’s a problem for the alliance. Without a VFA, the U.S. military would not be able to support either of these defense agreements.

My research explains how the VFA itself is a product of past alliance contention. In 1991, a more nationalist Philippine Senate voted not to renew a mutual basing agreement. Their decision led to the closure of Naval Base Subic Bay, effectively forcing the withdrawal of all U.S. troops from the Philippines. However, heightened security concerns in the South China Sea in the mid-1990s, as well as the slow pace of modernization of the armed forces of the Philippines, prompted Manila to revitalize defense ties with Washington by signing the VFA in 1998.

The Philippine Senate ratified the VFA in 1999. To avoid the impression of permanently stationing U.S. troops in the Philippines, the Senate emphasized the “visiting” and “temporary” status of U.S. forces, in keeping with their 1991 decision to abolish U.S. bases in the Philippines.

Duterte had previously targeted the VFA

In 2016, Duterte threatened to abrogate the VFA when the Obama administration delayed approval of U.S. aid to the Philippines in response to extrajudicial killings tied to Duterte’s “war on drugs.” The Philippine Drug Enforcement Agency officially reported 5,526 deaths from 2016-2019 as a result of police counternarcotics operations. However, human rights groups estimate up to 27,000 deaths stemming from wider violence.

Along with the United Nations, the European Union and international rights groups, the United States has attempted to hold the Duterte government accountable for violating human rights.

Following the guidelines of the Global Magnitsky Act, the U.S. Senate passed an amendment to an appropriations bill last December denying entry to individuals involved in the “wrongful imprisonment” of Philippine Sen. Leila de Lima — an outspoken critic of Duterte’s drug wars.

This was the likely reason the U.S. government canceled Sen. Ronald dela Rosa’s visa in late January. As national police chief, dela Rosa orchestrated Duterte’s counternarcotics policy from 2016 to 2018. Duterte did not take the news well, declaring he would terminate the VFA in response to the visa cancellation.

Duterte remains critical of the United States

Despite generally positive relations between the Philippines and the United States the past two decades, Duterte has remained ambivalent at best toward Washington. A populist with a temperament some analysts see as similar to that of President Trump, Duterte vowed to never travel to the “lousy” United States and revealed that he himself had once been denied a U.S. visa.

Duterte may have found a kindred spirit with Trump. However, their personal connection has not translated into strengthened bilateral ties. Instead, Duterte has openly courted Chinese investments while tilting his country away from Washington’s orbit, even against the recommendations of key advisers.

But scrapping the VFA won’t be easy to do

Will Duterte really end the VFA? By most accounts, he remains serious in his quest to eliminate the agreement. However, here’s why the VFA, and the U.S.-Philippine alliance, may weather Duterte’s firestorm.

First, the Philippine security establishment still values the alliance. The armed forces of the Philippines continues to benefit from the VFA, receiving military assistance, training, education and weapons. And although the Philippine secretary of foreign affairs and secretary of defense have mentioned the need to review the agreement and develop a self-sufficient national defense, neither have openly called for an end to the VFA. Key Philippine legislators have also appealed to Duterte to reconsider his decision on the VFA.

Second, it remains unclear whether the president has the constitutional authority to abrogate an international agreement ratified by the Philippine Senate. Senators remain divided on whether Duterte can unilaterally terminate the VFA and have even suggested that the Supreme Court weigh in on the legality of Duterte’s decision.

And third, the U.S. alliance and the VFA remain important to Philippine national security. In contrast to high degrees of trust and support for the United States, Filipinos hold much more negative attitudes toward China and remain wary of Beijing’s actions in the South China Sea. The Philippine public has also soured on Chinese foreign investment. The U.S.-Philippine alliance and the VFA therefore act as an insurance policy against Chinese threats.

Given widespread support for the alliance, Philippine political leaders may want to avoid triggering a major foreign policy crisis as they gear up for the next presidential election in 2022. Although elections remain more than two years away, Philippine politicians learned from their earlier rejection of U.S. bases that damage to the alliance takes several years to restore. In short, high diplomatic and security costs outweigh the political benefit of ending the VFA.

Manila and Washington now have 180 days to renegotiate the VFA before it expires. Periodic criticism of the VFA from civil society groups may provide Duterte and his allies some political cover to adjust the terms of the agreement. However, the Philippine president will find it difficult to permanently eliminate the VFA because of domestic resistance and broader strategic concerns.

Andrew Yeo is an associate professor of politics at the Catholic University of America in Washington, D.C., and is currently a Fulbright visiting research fellow at the University of the Philippines Diliman. His most recent book isAsia’s Regional Architecture: Alliances and Institutions in the Pacific Century (Stanford University Press, 2019).