By Dominic Casciani
BBC News at the National Archives
Being allies in wartime is a difficult business. Just ask Defence Secretary Geoff Hoon who this week told MPs that Britain would have failed in its duty as an ally if it does not send British troops to assist Americans in a dangerous part of Iraq.
U-boats relied on Enigma communications
But documents revealed at the National Archive reveal allies often fall out at the most crucial of times - and in the case of wartime code-breaking, Britain believed it was in the right to give Washington the brush off.
The UK's code breakers at Bletchley Park were instrumental to the winning of World War II.
Despite a recent reinterpretation of history by Hollywood, the team, which included mathematical genius Dr Alan Turing, the father of computing, were crucial to breaking Nazi Germany's codes.
Their biggest and most important breakthrough was cracking the Enigma machine, the encoding engine, which rendered communications indecipherable without access to the technology.
But while the Americans made their own technical leaps forward, not least with the invention of the atom bomb, tensions mounted over what the code breakers were prepared to share with US colleagues.
By October 1941 Dr Turing's team had worked out that Enigma traffic was split into a number of zones and then further split depending on the type of message being sent.
Adding to the challenge, completely different encryption methods were used for messages within Germany, between the Nazi high command and between Berlin and axis partner Rome.
The team at Bletchley Park desperately needed access to American advances down the road towards computing.
But Washington was convinced London was withholding information that could prove essential to its own war effort.
In November 1941 an urgent telegram from Washington reached the attention of Sir Stewart Menzies, the chief of the secret service.
Adm Leigh Noyes, head of US Navy communications claimed London had gone back on a deal of free exchange of information between the code-breakers.
"He states they are aware that you hold certain European code books and keys which he claims by virtual agreement should be imparted to Washington for their use," Sir Stewart was told.
"Noyes is in a mood to withhold further information unless he receives full reciprocal information on European work."
In short, military co-operation in this sphere of the war was short on the ground.
But London stood firm. It stressed it was passing all it had of use to Washington - but would not pass over material that was not apparently relevant to the American cryptographers.
Adm Noyes retorted only the cryptographers could be in a position to judge what was important, and his team were making great leaps forward in mechanical answers to code-breaking.
The Enigma machine rendered communications indecipherable
The row spilled over into 1942 with London apparently insisting it was doing all it could. However a separate internal memo revealed Washington had not been told of the capture of an Enigma unit from a U-boat.
In an attempt to calm the waters, Dr Turing himself was sent to the US to see what the teams could learn from each other.
Despite public knowledge of the code-breakers since the 1970s, Dr Turing's report has remained secret until now. It goes into rich detail of how the operations furiously worked to break the Nazi communications system.
But amid the mass of technical data, his conclusion on American efforts was clear - they were not up to the task.
Dr Turing said: "Generally speaking, their attitude is so purely mechanical and mathematical that they often fail to see the wood for the trees and do not like to admit that experience and a knowledge of immediately prior developments, combined with a little manual work, may often produce the answer more quickly than machinery.
"I am persuaded that one cannot very well trust these people where a matter of judgement in cryptography is concerned."
He asked to bring all the new American technology back to the UK, believing his team could make much better use of the machines. His proposal was supported and his team eventually built Colossus, one of the world's first programmable machines.