Title: United Kingdom–United States relations
Content:  Relations between the United Kingdom and the United States have ranged from two early wars to competition for world markets. Since 1940 the countries have been close military allies enjoying the Special Relationship built as wartime allies and NATO partners. They are bound together by shared history, an overlap in religion, common language, legal system and kinship ties that reach back hundreds of years, including kindred, ancestral lines among English Americans, Scottish Americans, Welsh Americans, Cornish Americans, Scotch-Irish Americans, Irish Americans, and American Britons. Today, large numbers of expatriates live in both countries. In the early 21st century, Britain affirmed 
Title: An Ocean Apart
Content:  During the 1980s, the United States and Britain form an alliance against the Soviet Union.
Title: United Kingdom–United States relations
Content:  advocating the invasion of Iraq in 2003. Again Britain was second only to the US in sending forces to Iraq. Both sides wound down after 2009, and withdrew their last troops in 2011. President Bush and Prime Minister Blair provided sustained mutual political and diplomatic support and won votes in Congress and parliament against their critics at home. During this period Secretary of State Donald Rumsfeld said that "America has no finer ally than the United Kingdom." The 7 July 2005 London bombings emphasised the difference in the nature of the terrorist threat to both nations. The United States concentrated primarily on global enemies, like the al-Qaeda network and other Islamic extremists from the Middle East. The London 
Title: Special Relationship
Content:   The intense level of military co-operation between the UK and the US began with the creation of the Combined Chiefs of Staff in December 1941, a military command with authority over all American and British operations. After the end of the Second World War, the joint command structure was disbanded, but close military cooperation between the nations resumed in the early 1950s with the start of the Cold War.
Title: Special Relationship
Content:  Relationship" between the UK and the US was perhaps most memorably emphasized by Churchill, its existence and even the term itself had been recognized since the 19th century, not least by rival powers. Existence in the 19th century and early 20th century: The American and British governments were enemies when foreign relations between them first began, after the American colonies declared their independence from British rule, which triggered the American Revolutionary War. Relations often continued to be strained until the mid-19th century, erupting into open conflict during the War of 1812 and again verging on war when Britain almost supported the rebel Confederate States 
Title: United Kingdom–United States relations
Content:  When the United States became the world's lone superpower after the dissolution of the Soviet Union, new threats emerged which confronted the United States and its NATO allies. With military build-up beginning in August 1990 and the use of force beginning in January 1991, the United States, followed at a distance by Britain, provided the two largest forces respectively for the coalition army which liberated Kuwait from Saddam Hussein's regime during the Persian Gulf War. In the 1997 general election, the British Labour Party was elected to office for the first time in eighteen years. The new prime minister, Tony Blair, and Bill Clinton both used the expression "Third Way" to describe their centre-left ideologies. In August 1997, the American people expressed solidarity with the British people, sharing in their grief and sense of shock on the death of Diana, Princess of Wales, who perished in a car crash in Paris. Throughout 1998 and 1999, the United States and Britain sent troops to impose peace during the Kosovo War. Tony Blair made it a point to develop very close relationships with the White House.
Title: United Kingdom–United States relations in World War II
Content:  The UK-US relations in World War II comprised an extensive and highly complex relationships, in terms of diplomacy, military action, financing, and supplies. British Prime Minister Winston Churchill and American President Franklin D. Roosevelt formed close personal ties, that operated apart from their respective diplomatic and military organizations.
Title: Allies of World War II
Content:  Before they were formally allied, the United Kingdom and the United States had cooperated in a number of ways, notably through the destroyers-for-bases deal in September 1940 and the American Lend-Lease program, which provided Britain and the Soviet Union with war materiel beginning in October 1941. The British Commonwealth and, to a lesser extent, the Soviet Union reciprocated with a smaller Reverse Lend-Lease program. The First Inter-Allied Meeting took place in London in early June 1941 between the United Kingdom, the four co-belligerent British Dominions (Canada, Australia, New Zealand and South Africa), the eight governments in exile (Belgium, Czechoslovakia, Greece, Luxembourg, the Netherlands, Norway, Poland, Yugoslavia) and Free France. The 
Title: Russia–United Kingdom relations
Content:  Russia–United Kingdom relations, also Anglo-Russian relations, involve the bilateral relationship between Russia and the United Kingdom. Formal ties between the courts started in 1553. Russia and Britain became allies against Napoleon in the early-19th century. They were enemies in the Crimean War of the 1850s, and rivals in the Great Game for control of central Asia in the latter half of the 19th century. They allied again in World Wars I and II, although the Russian Revolution of 1917 strained relations. The two countries were at sword's point during the Cold War (1947–1989). Russia's big business tycoons developed strong ties with London financial institutions in the 1990s after the dissolution of the USSR in 1991. The countries share a history of intense espionage activity against each other, with the Soviet Union succeeding in penetration of top echelons of the British 
Title: United Kingdom–United States relations
Content:  was decaying and Johnson disliked all diplomats. London, furthermore, relied less and less on ambassadors and embassies. Through the US-UK Mutual Defence Agreement signed in 1958, the U.S. assisted the UK in its development of a nuclear arsenal. The British, however, were financially unable to develop their own delivery systems for nuclear weapons. In April 1963, the Polaris Sales Agreement called for the U.S. to sell the UGM-27 Polaris ballistic missile for use in the Royal Navy's submarine fleet starting in 1968. The American containment policy called for military resistance to the expansion of communism, and the Vietnam War became the 

Question: when did the uk and us become allies?
Answer: Since 1940.
1940.