Historical background.
(Compiled from documents in the US National Archive.)

1942
January 1942 Start of U Boat offensive against US East coastal shipping. U Boats operated singularly so very little radio communications.
1st February 1942 U Boat Shark key went over to M4 Enigma. Bletchley Park blacked out.
19th April 1942 Rodger Winn in America. Met Admiral Richard S Edwards. When Winn left in May, US Navy U Boat tracking room up and running. But little info coming from Uk because of M4 blackout. US thought Brits were holding back Enigma info.
April, Tiltman sent to America
13th May 1942 Travis informed OP-20-G policy agreed for E solutions
3rd June 1942 Travis proposed regulations for use of ULTRA
7th June 1942 newspaper stories. "Navy had word of Jap plan to strike at sea"
1st July 1942 Robert Ely and Joseph Eachus arrive in UK given complete wiring diagrams and blueprints of Bombe
August 1942 Wenger, head of OP-20-G, convinced UK concealing success against M4. Wrong. M4 still not broken.
3rd September 1942 Wenger proposed programme to develop US 4 wheel Bombe
September 1942. Travis and Birch to Washington. Agreed full collaboartion. exchange traffic, recovered keys and cribs.
29th October 1942 U-559 captured. Documents recovered.
Late 1942 Turing to Washington to consult with US Navy on Bombe design
13th December 1942 M4 broken using Short Weather Codes Codes captured from U-559
15th December 1942 US Army sign contract with AT&T for 003 Bombe

1943
4th January 1943 TItman and Turing told about 003, Madam X.
5th February 1943 Turing shown Madam X
3rd May 1943 first two US Bombes begin tests at Dayton
17th May 1943 BRUSA agreement signed
3rd April 1943 William Friedman visit to London and Bletchley Park
22nd June 1943 First results from US Navy Bombes sent to GC&CS

The arrival of the Americans
August 1943 first contingent of Signal Corps left Washington for Bletchley Park.
1st February 1944 American 6812th Signal Security Detachment set up to run Uk Bombes at Eastcote.
6811th manned intercept station at Bexley in Kent in Southern England.
29th March 1944 6813th Signal Security Detachment in Bletchey Park.
The American 6813th Signal Security Detachment.

In early 1942 Lt Colonel (then Captain) S. Kullback toured Bletchley Park, paving the way for the visit of Lt. Colonel (then Captain) Johnson later in the same year. Based upon the study and recommendations of Capt. Johnson and with the approval of the British Foreign Office which controls G. C. and C. S., the decision was made to send American Signal Corps personnel to participate in the activities at Bletchley Park.

The first contingent left Washingtom in August 1943 and the last major party, departed in March 1944. In late February 1944 the 6813th Signal Security Detachment was organised and on 29 March 1944 a post opened in the Manor House at Little Brickhill (Buckinghamshire). The then Captain Bundy served as the first commanding officer and remained in that capacity throughout the operational history of the detachment.

The Americans were fully integrated into all parts of the Bletchley Park operations. Hut 6, by now in D Block, where German Army and Air Force Enigma was being broken. Hut 3 in D Block doing intelligence extraction from the decrypts. Sixta, doing Traffic Analysis and into the Newmanry and Testery breaking the Lorenz ciphers in F Block and H Block.

An American's view
"Bletchley Park was (and is) an amazing institution even to those old hands among the Americans, now rendered less awe-struck by the hardening effect of two or more years spent within its precincts. At first exposure complete chaos seemed to reign overall but with acquaintance beneath the superficial confusion order found itself---not that order which would have been susceptible of being rendered diagrammetically by an efficiency expert but a pragmatic system constantly changing and constantly adapting itself to a problem which refused to remain stationary."

The 6813th Signal Security Detachment had an average strength of 85 officers and men, 20 of these serving in administrative capacity to maintain the post at Little Brickhill and the remaining 65 in various assignments at G. C. and C. S.

6812TH SIGNAL SECURITY DETACHMENT APO 413 US ARMY

15 June 1945
The following is a brief description of the operations of the 6812th Signal Security Detachment in the European Theater of Operations from 1 February 1944 to 7 May 1945.

It was necessary to completely train the operators, engineers and technicians before the work could begin. The contents of the following pages had to be thoroughly understood by each man.

The 6812th reached a degree of efficiency far above the greatest expectations. When operations ceased on VE day the Detachment was averaging in output runs per day approximately 38 % above that of the British units engaged in the same work. This seems almost incredible when it is understood that the British had the benefits of four years of experience. From the beginning of operations to VE Day the 6812th Signal Security Detachment found the solutions to a total of 425 German Enigna Keys.

About 290 Americans in the UK in 6811,12 and 13.