LONDON, July 5 (Reuters) - Britain's so-called 'special relationship' with the United States was one of the most enduring alliances of the 20th Century, though Brexit and the election of U.S. President Donald Trump have raised questions about its future.
What is the special relationship and how do the United States and Britain compare on key measures?
ECONOMY
The United States is the world's biggest economy, worth about $20.4 trillion or 23 percent of global GDP, while Britain is the world's fifth largest, worth about $2.9 trillion or 3.3 percent of global GDP.
While the EU accounts for about half of Britain's external trade, the United States is by far the biggest single trading partner, followed by Germany, the Netherlands, France and China.
For the United States, Britain is its seventh largest trade partner, after China, Canada, Mexico, Japan, Germany and South Korea. Both Britain and the United States report a trade surplus in goods with each other, indicating they are not comparing like with like.
For decades, both Washington and London were aligned on opening up world markets to free trade. British Prime Minister Theresa May said U.S. steel and aluminium tariffs were "unjustified and deeply disappointing".
MILITARY
The United States, which spends more on its military than the next seven highest-spending countries combined, has a defence budget proposal for 2018 of $639.1 billion. It has the second largest number of nuclear weapons after Russia.
The U.S. Navy operates a fleet of 14 Ohio-class nuclear submarines, each of which can carry 20 Trident II D5 missiles which have multiple, independently-targeted warheads.
Britain, the biggest defence spender in the EU, spends about 40 billion pounds ($53 billion) a year. France and Russia are the only other nuclear powers in Europe.
Britain has a fleet of four Vanguard-class submarines which can each carry 16 Trident II D5 missiles, which are leased from a pool of such missiles that it shares with the U.S. Navy.
Their militaries have fought side-by-side in Europe, Korea, Kuwait, Iraq, the former Yugoslavia, Afghanistan, Libya and Syria. Britain views the U.S.-led NATO alliance as the foundation of its defence.
Trump has demanded Western allies pay more into NATO. The United States operates out of several military bases in Britain including RAF Lakenheath, RAF Croughton and RAF Menwith Hill.
INTELLIGENCE
Intelligence cooperation between the United States and Britain is extremely close and one of the main areas where the alliance works in practice.
During World War Two, the two countries forged a close intelligence sharing deal that was later extended to include Canada, Australia and New Zealand and became known as the "Five Eyes" (FVEY).