The British intercepted the Zimmermans telegram
It was Germany, during WWI. On January of 1917, the Foreign Minister of Germany, Arthur Zimmerman, sent a telegram to the German ambassador in Mexico making this proposal. However, it was intercepted by British intelligence and forwarded to the US government. This contributed to the US entering the war.
Germany sent Mexico a document trying to get them to declare war on the USA and thereby prevent USA from entering the war on the side of the Allies. The British intercepted the document and after they showed it to the Americans, the USA declared war on Germany, entering WWI.
No, there is no British MI4. The British intelligence agency commonly known as MI6 (Secret Intelligence Service) is responsible for foreign intelligence and counterintelligence operations. There is no official mention of an MI4 in the British intelligence community.
Arthur Zimmerman (1864 - 1940)
When the British laid siege, they also intercepted the fur trade routes.
Zimmermann telegram
Zimmermann telegram
Bruce Mackenzie - British intelligence operative - died in 1978.
the zimmerman note.
Most probably, you are referring to an alliance between Mexico and Germany during WW1. It was Arthur Zimmerman, German ambassador in Mexico. His "Zimmerman note" was intercepted by British intelligence and forwarded to the American government, hastening American entry into WW1.
To "distract" the US from entering WW1. Such proposal was however, discovered by British intelligence forces and forwarded to the United States, hastening the entry of the United States into WW1, against Germany.
British Intelligence - 1940 was released on: USA: 29 January 1940 France: 31 July 1946