The United States has formal mutual defense treaties with more than 50 countries. US allies fall into three treaty groupings: the 32-nation NATO alliance, five bilateral treaty partners in the Indo-Pacific, and the Rio Treaty signatories in the Western Hemisphere. A separate legal category, Major Non-NATO Ally, adds 19 more countries that receive defense trade benefits without any promise of American military protection.
What Counts as an Ally
In U.S. foreign policy, “ally” has a specific meaning. It refers to a country linked to the United States by a Senate-ratified treaty that includes a mutual defense commitment. Both sides are legally obligated to respond if the other is attacked. That is different from a strategic partnership, a security dialogue, or an intelligence-sharing arrangement, which involve cooperation but carry no binding promise to fight alongside the other country.
The Department of Defense draws the same line. Alliances rest on treaties. Partnerships rest on shared interests and can be adjusted or ended without any legal process. Both matter, but only treaty allies have a formal claim on American military support.
NATO Members
The North Atlantic Treaty Organization is the largest U.S. alliance. Founded in 1949, it now has 32 members across North America and Europe.1NATO. NATO Member Countries Article 5 of the North Atlantic Treaty establishes that an armed attack on any member is treated as an attack on all of them, with each member deciding individually how to respond, up to and including armed force.2SHAPE. Invoking Article 5 Article 5 has been invoked exactly once, after the September 11, 2001, attacks on the United States.
The 32 NATO members are:
- Albania
- Belgium
- Bulgaria
- Canada
- Croatia
- Czechia
- Denmark
- Estonia
- Finland
- France
- Germany
- Greece
- Hungary
- Iceland
- Italy
- Latvia
- Lithuania
- Luxembourg
- Montenegro
- Netherlands
- North Macedonia
- Norway
- Poland
- Portugal
- Romania
- Slovakia
- Slovenia
- Spain
- Sweden
- Turkey
- United Kingdom
- United States
Finland joined in 2023 and Sweden in March 2024. Both ended decades of military non-alignment in response to Russia’s 2022 invasion of Ukraine.
Treaty Allies in the Indo-Pacific
Outside NATO, the United States has five separate mutual defense treaties in the Indo-Pacific. Each was negotiated individually, and each follows a similar structure: both sides agree that an armed attack on either would endanger the other, and each commits to respond according to its constitutional processes.3U.S. Department of State. U.S. Collective Defense Arrangements
Japan
The Treaty of Mutual Cooperation and Security, signed January 19, 1960, commits both countries to treat an armed attack on territories under Japanese administration as a threat to each country’s own safety.4United States Department of State. U.S. Security Cooperation With Japan The treaty also provides the legal basis for U.S. bases in Japan, which host more American military personnel than any other foreign country.
South Korea
The Mutual Defense Treaty with the Republic of Korea, signed October 1, 1953, came out of the Korean War. It commits both countries to respond to an armed attack in the Pacific on either party.5The American Presidency Project. Special Message to the Senate Transmitting the Mutual Defense Treaty Between the United States and the Republic of Korea Roughly 28,500 U.S. troops are stationed in South Korea.
The Philippines
The Mutual Defense Treaty with the Republic of the Philippines, signed August 30, 1951, was one of the earliest U.S. security commitments in the Pacific. Under Article IV, each country recognizes that an armed attack in the Pacific area on either party would endanger both and agrees to act through its constitutional processes.6Avalon Project – Lillian Goldman Law Library. Mutual Defense Treaty Between the United States and the Republic of the Philippines
Australia (ANZUS)
The Australia, New Zealand, and United States Security Treaty, signed September 1, 1951, commits each party to treat an armed attack in the Pacific on any member as a threat to all.7The Avalon Project. Security Treaty Between the United States, Australia, and New Zealand (ANZUS) There is a wrinkle worth knowing. In 1986, the United States suspended its defense obligations toward New Zealand after New Zealand banned nuclear-armed and nuclear-powered ships from its ports. Relations were formally restored through the 2010 Wellington Declaration and a 2012 lifting of military cooperation restrictions. The U.S.-Australia leg has always been the most active of the three.
Thailand
Thailand’s alliance rests on the Southeast Asia Collective Defense Treaty, known as the Manila Pact, signed September 8, 1954. That treaty created SEATO, which dissolved in 1977, but the underlying treaty was never terminated. Under Article IV, each party agrees that armed aggression in the treaty area against any member would endanger its own safety and commits to act through its constitutional processes.8The Avalon Project – Yale Law School. Southeast Asia Collective Defense Treaty (Manila Pact) The United States attached an understanding at signing that its obligation applied specifically to communist aggression, with a commitment to consult on other threats.
The Rio Treaty in the Western Hemisphere
The Inter-American Treaty of Reciprocal Assistance, known as the Rio Treaty, is the oldest U.S. mutual defense agreement still in force. Signed September 2, 1947, it predates NATO by two years. Its core obligation mirrors Article 5: an armed attack against any American state is treated as an attack against all, and each signatory commits to help meet it.9Organization of American States (OAS). Inter-American Treaty of Reciprocal Assistance
In practice the Rio Treaty carries less weight than NATO or the Indo-Pacific treaties. Mexico, Ecuador, Venezuela, Bolivia, and Nicaragua have withdrawn over the years. It remains a legally binding mutual defense commitment between the United States and roughly a dozen Latin American and Caribbean nations.3U.S. Department of State. U.S. Collective Defense Arrangements
Major Non-NATO Allies
Major Non-NATO Ally (MNNA) status is a legal designation, not an alliance. It carries no promise that the United States will defend the country. What it does provide is a package of defense trade and cooperation benefits, established under 22 U.S.C. ยง 2321k, which requires the President to notify Congress at least 30 days before granting or revoking it.10Office of the Law Revision Counsel. 22 U.S.C. 2321k – Designation of Major Non-NATO Allies
The practical benefits include eligibility for cooperative research and development projects with the Department of Defense on a cost-sharing basis, priority delivery of military surplus equipment, the ability to host U.S. war reserve stockpiles, and the right of their defense firms to bid on certain DoD maintenance contracts abroad.11Office of the Law Revision Counsel. 10 U.S. Code 2350a – Cooperative Research and Development Agreements
Nineteen countries currently hold MNNA status:10Office of the Law Revision Counsel. 22 U.S.C. 2321k – Designation of Major Non-NATO Allies
- Argentina
- Australia
- Bahrain
- Brazil
- Colombia
- Egypt
- Israel
- Japan
- Jordan
- Kenya
- Kuwait
- Morocco
- New Zealand
- Pakistan
- Philippines
- Qatar
- South Korea
- Thailand
- Tunisia
Several of these countries, including Australia, Japan, South Korea, the Philippines, and Thailand, are also formal treaty allies. MNNA status adds defense procurement advantages on top of the treaty commitment.
Taiwan sits in a unique position. Federal law directs that Taiwan be treated as though it were a Major Non-NATO Ally for purposes of defense equipment and service transfers, without formally carrying the designation.10Office of the Law Revision Counsel. 22 U.S.C. 2321k – Designation of Major Non-NATO Allies
The designation is not permanent. Afghanistan was named a Major Non-NATO Ally in 2012, and President Biden terminated that designation on September 23, 2022, following the Taliban takeover.12Federal Register. Terminating the Designation of Afghanistan as a Major Non-NATO Ally
Partnerships That Are Not Alliances
Some high-profile security groupings often mentioned alongside treaty allies are not alliances at all. They involve no binding defense obligation.
AUKUS is a trilateral security partnership among Australia, the United Kingdom, and the United States, announced in September 2021. Its most visible project helps Australia acquire conventionally armed, nuclear-powered submarines, with a second track focused on joint development of advanced military capabilities.13United States Department of State. About Us – Office of the AUKUS Senior Advisor It is not a treaty and contains no mutual defense clause.
The Quad brings together Australia, India, Japan, and the United States as a diplomatic partnership focused on a free and open Indo-Pacific.14Australian Government Department of Foreign Affairs and Trade. The Quad It has no treaty, no mutual defense commitment, and no military command structure. India is not a U.S. treaty ally.
Five Eyes is an intelligence-sharing arrangement among Australia, Canada, New Zealand, the United Kingdom, and the United States, rooted in World War II signals intelligence cooperation. It involves no mutual defense pledge.
How Alliances End
The Constitution says how treaties are made (negotiated by the President, ratified by two-thirds of the Senate) but says nothing about how they end.15Constitution Annotated. Breach and Termination of Treaties Practice has been inconsistent. Sometimes Congress has authorized a President to give notice of withdrawal; sometimes Presidents have acted alone. When President Carter terminated the mutual defense treaty with Taiwan in 1979, the Supreme Court dismissed a challenge in Goldwater v. Carter without reaching the merits, leaving the question unresolved.
Congress addressed NATO specifically. The National Defense Authorization Act for Fiscal Year 2024 prohibits any President from withdrawing the United States from NATO without approval of two-thirds of the Senate or an act of Congress. Whether that provision would survive a constitutional challenge has not been tested.
For Major Non-NATO Ally designations, the process is simpler. The President can terminate MNNA status with 30 days’ notice to Congress. No Senate vote or legislation is required.10Office of the Law Revision Counsel. 22 U.S.C. 2321k – Designation of Major Non-NATO Allies