14 documented events, earliest first. Every card carries its evidence tier and its own sources.
September 2, 1940
Destroyers for Bases Agreement
The United States transferred fifty aging destroyers to the Royal Navy in exchange for 99-year rent-free leases on British base sites in Newfoundland, Bermuda, the Bahamas, Jamaica, Antigua, St Lucia, Trinidad and British Guiana. Executed by executive agreement rather than treaty, which avoided a Senate vote while the US was formally neutral.
Fifty destroyers for eight base sites on 99-year leases — a term longer than the ships, most of which were First World War vintage.
What it changed
Established the pattern that recurs through the whole relationship: Britain trades territory, position or autonomy for American equipment, at a moment when it cannot decline.
Franklin D. RooseveltWinston Churchill
March 11, 1941
Lend-Lease Act
Signed into law, allowing the President to supply materiel to any country whose defense he deemed vital to the United States. It ended the cash-and-carry constraint that had been draining British reserves, and made the United States the arsenal of the British war effort.
What it changed
Britain fought the rest of the war on American supply. The abrupt termination of Lend-Lease in 1945 is what forced the loan negotiation below, so this act and the 1946 debt are one continuous event.
Franklin D. Roosevelt
August 14, 1941
Atlantic Charter
Issued after a shipboard meeting at Placentia Bay, Newfoundland — four months before the United States entered the war. A statement of war aims including self-determination, free trade and freedom of the seas, signed by a belligerent and a neutral.
What it changed
Its self-determination language was later read against the British Empire itself, by colonial independence movements and by American officials. Britain signed a document that helped set the terms of its own decolonization.
August 19, 1943
Quebec Agreement — merged nuclear programs
A secret agreement merging the British Tube Alloys project into the Manhattan Project. Each side undertook not to use the weapon against the other, not to use it against third parties without the other's consent, and not to pass information to third parties without consent.
What it changed
British scientists contributed to a weapon that became American property. The consent provisions were superseded three years later by US domestic law, which is the point of the 1946 rupture below.
Winston ChurchillFranklin D. Roosevelt
March 5, 1946
BRUSA / UKUSA signals intelligence agreement
A secret agreement between the UK's London Signals Intelligence Board and the US State–Army–Navy Communication Intelligence Board to share signals intelligence and, in effect, not to target each other. It built on an earlier British–US communication intelligence agreement of May 17, 1943.
Signed for the UK by Colonel Patrick Marr-Johnson and for the US by Lieutenant General Hoyt Vandenberg. The text was not publicly released until 2010.
What it changed
The foundation of what became Five Eyes. Canada, Australia and New Zealand were brought in by 1955. This is the one major structure on this page that is genuinely reciprocal — and the one that stayed secret longest.
Patrick Marr-JohnsonHoyt Vandenberg
Effective July 15, 1946
Anglo-American Loan
After Lend-Lease was terminated, Britain borrowed US$3.75 billion from the United States at 2% interest, with a further US$1.19 billion from Canada. The terms included a commitment to make sterling convertible, an attempt at which collapsed within weeks in 1947.
The final payment was made on December 29, 2006 — about US$83 million (£45.5 million) to the United States and US$23.6 million (£12 million) to Canada. Total repaid, with interest: roughly US$7.5 billion. It finished six years late because Britain suspended payments in 1956, 1957, 1964, 1965, 1968 and 1976.
- ·The 1956 suspension falls in the year of Suez — the same exchange-rate pressure appears in both places on this page.
- ·December 29 was chosen simply as the last working day of the year.
What it changed
Britain entered the postwar order as a debtor to its own ally, with a repayment schedule that ran for sixty years and a currency commitment it could not meet.
July 3, 1958
US–UK Mutual Defense Agreement
Restored the nuclear cooperation the 1946 Atomic Energy Act had blocked, permitting transfer of nuclear materials, technology and weapon design information. Britain had by then tested its own thermonuclear device, which is what made the United States willing to reopen the channel.
What it changed
Britain's deterrent has been developed inside an American technical relationship ever since. The agreement has been renewed repeatedly and remains the legal basis for that cooperation.
The sequence is worth noting: Britain regained access by demonstrating it could proceed without it.
April 6, 1963
Polaris Sales Agreement
Following the Nassau meeting of December 1962, the United States agreed to sell Polaris missiles to Britain. Britain built the submarines and the warheads; the missiles came from the United States.
What it changed
Set the division of labor that still governs the British deterrent: British boats, British warheads, American missiles. The Trident arrangement below is its direct descendant.
John F. KennedyHarold Macmillan
1965–1973
British Indian Ocean Territory and the Chagos removal
Britain detached the Chagos Archipelago from Mauritius in 1965 to create the British Indian Ocean Territory, then agreed by exchange of notes in 1966 to make it available for US defense purposes. The resident Chagossian population was removed between 1968 and 1973 so that Diego Garcia could become a US base.
What it changed
Gave the United States a major base in the Indian Ocean on British-administered territory, used for operations across the Middle East and Central Asia for the following half-century. It also produced a sovereignty dispute that took sixty years to settle — see the 2025 entry.
Population figures for the removal vary by source and by how the count treats contract workers; the archipelago's population was in the low thousands. The removal itself is not disputed.
April–June 1982
US support in the Falklands War
After an initial attempt at mediation between two American partners, the United States supported Britain materially — including supply of the latest AIM-9L Sidewinder air-to-air missile, fuel and use of facilities at Ascension Island, and intelligence.
What it changed
Demonstrated that when forced to choose, the United States chose the UK over a hemispheric partner. It is the clearest case on this page of the relationship paying out in Britain's direction.
Ronald ReaganMargaret ThatcherCaspar Weinberger
April 15, 1986
Operation El Dorado Canyon — US strikes on Libya from UK bases
US F-111 aircraft flew from bases in England to strike targets in Libya. France and Spain refused overflight, forcing a long route around the Iberian peninsula with multiple refuelings. Britain granted permission; the decision was politically costly domestically.
What it changed
Established in practice that US bases in Britain can be used for operations Britain is not party to, with British consent as the only constraint. That consent requirement is a convention of the basing arrangement rather than a published treaty term.
Ronald ReaganMargaret Thatcher
Signed March 31, 2003; in force April 26, 2007
US–UK Extradition Treaty
Replaced the 1972 treaty. Widely criticized in Britain as asymmetric: extradition from the UK to the US requires reasonable suspicion, while a UK request to the US is tested against the US constitutional probable-cause standard.
The independent review by Sir Scott Baker concluded there is no significant difference between the probable cause and reasonable suspicion tests, and the Home Affairs Committee found little or no distinction in practice. The asymmetry in the text is real; whether it changes outcomes is contested.
What it changed
Became the standing example in British politics of an unequal bilateral instrument, and the frame through which later extradition cases were argued.
Included with the counter-finding attached. The criticism is well documented and so is the review that qualifies it; presenting only the first would be the easier story and the wrong one.
September 15, 2021
AUKUS announced
A trilateral security partnership between Australia, the UK and the US. Its first pillar is the provision of nuclear-powered submarines to Australia; the second covers advanced capabilities including quantum, AI and undersea systems.
What it changed
Extended the Anglo-American nuclear propulsion relationship to a third country, and cost France a submarine contract — the 2021 entry in a long record of this alliance being built at another ally's expense.
Signed May 22, 2025
UK–Mauritius treaty on the Chagos Archipelago
The UK agreed to transfer sovereignty over the Chagos Archipelago to Mauritius while retaining rights over Diego Garcia under an initial 99-year lease covering the island and the waters within 12 nautical miles. Mauritius may arrange Chagossian resettlement on every island except Diego Garcia. Negotiations began in November 2022.
A 99-year term — the same length as the base leases Britain granted the United States in 1940. The instrument that opens this page and the one that closes it use the same number.
What it changed
Resolves a sovereignty dispute created in 1965 without disturbing the base created in 1966. The territory changes hands; the American facility does not.
Recent and still moving: parliamentary scrutiny and ratification extended past the signing date. Treat the signing and the terms as established and the implementation status as open.