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US–Japan alliance
Treaty 1951, revised 1960· Alliance or organization
FactTreaties, membership, measured data, events.
Japan's security treaty with the US: Washington defends Japan, and Japan hosts the largest US troop presence abroad.
See it on the Geopolitics map →The picture
The first treaty was signed in San Francisco on September 8, 1951, the same day as the peace treaty ending the occupation. The 1960 revision, passed despite mass protests, commits the US to defend territory Japan administers (Article 5), including, Washington says, the Senkaku Islands China claims; Japan provides bases (Article 6). About 55,000 US troops, the Seventh Fleet at Yokosuka and Marines on Okinawa are based there. Since 2015 Japan may help defend allies, within limits.
Why it matters
It anchors US power in Asia and is Japan's main deterrent against China and North Korea, letting Japan rearm within its pacifist constitution rather than alone.
Key facts and numbers
The 1951 and 1960 treaty texts, the 2015 defense guidelines, US–Japan '2+2' joint statements and Ministry of Defense white papers; troop numbers from the Pentagon.
What to watch
Debated: how far Japan would join a Taiwan war, how much more Tokyo should pay under Trump, and Okinawans' long opposition to hosting most US bases.
Connections (3)
▸Japan is defended under US–Japan allianceFact
The relationship
Japan's 1947 constitution renounces war, so the alliance gives it a US security guarantee and nuclear umbrella while it hosts US forces and pays about ¥211 billion a year toward their costs. Since 2022 Tokyo has leaned in further: rising defense spending, long-range missiles and a new joint command to work closely with US forces.
Since when and why
Tokyo and Washington; Article 9 of the 1947 constitution; the current five-year host-nation support deal runs through March 2027.
How we know
The treaty texts, the Special Measures Agreement on host-nation support, and Japan's December 2022 security strategy documents.
What to watch
Debated: whether Washington would actually fight China over remote islands, and how much more Trump will demand in the next cost-sharing talks.
▸Japan's defense buildup strengthens US–Japan allianceWidely agreed
The relationship
Japan's buildup shifts the alliance from 'US spear, Japanese shield' toward shared roles. Japan is buying US Tomahawks for counterstrike, created a Joint Operations Command and co-produces missiles with the US, while US Forces Japan is being upgraded into a joint command to work alongside it.
Since when and why
Agreed at US–Japan '2+2' meetings of defense and foreign ministers in 2023–24; Japan's Joint Operations Command opened in March 2025.
How solid?
US–Japan joint statements, Pentagon and Japanese Defense Ministry announcements, and US State Department approval of the Tomahawk sale.
What to watch
Whether Washington demands more, such as 3.5% of GDP, and higher host-nation payments when the current cost-sharing deal expires in March 2027.
▸Camp David US–Japan–South Korea summit added a trilateral layer to US–Japan allianceFact
The relationship
Camp David added a three-way layer on top of the US–Japan treaty: Tokyo agreed to consult Seoul and Washington in crises and to share missile-warning data in real time. It created no new defense treaty; Japan's US guarantee is unchanged, but Japanese forces now train regularly with South Korea's.
Since when and why
Camp David, Maryland, August 18, 2023; the first Freedom Edge exercise followed in June 2024.
How we know
The published summit documents and Japanese Defense Ministry announcements of the data link and drills.
What to watch
Whether shifting politics in Tokyo and Seoul, and unresolved history disputes, limit how far the trilateral layer goes.