MI5 and the FBI Evolved Alongside One Another, But Their Relationship Hasn’t Always Been Constructive

Since its creation in 1909 as the Home Department of the Secret Service Bureau, and its designation during World War I as Section 5 of the War Office’s military intelligence directorate, the organization now known simply as MI5 has carved out an independent niche unique in Whitehall.

Headed at its beginning by Vernon Kell — one of Britain’s longest-serving civil servants, if not the longest-serving ever — MI5 from the start exercised its authority to initiate investigations without any political guidance. Kell interviewed his staff personally, and refused to employ Roman Catholics, asserting that the Pope’s intelligence network was the best in the world, and he wasn’t there to improve it. Such a tiny security apparatus looms disproportionately large across British society. The FBI, which serves a somewhat parallel role in the United States, had its own long-serving and controlling leader in J. Edgar Hoover. But while the two agencies have evolved alongside one another, in the service of nations that are longtime allies, their relationship has not always been a constructive one.

In 1938, MI5 had sought to develop a relationship with the FBI by offering information about a German spy in New York who was part of an extensive Nazi network. But Hoover at that point had no experience of counter-espionage operations, which often depend on patient surveillance for long periods, and he quickly arrested a U.S. Army deserter, Gunther Rumrich, without informing MI5 about the plan. While telling the FBI had been part of an effort to establish a permanent link between London and Washington, D.C., Hoover’s premature intervention compromised a wider round-up in Europe, and enriched an FBI special agent, Leon Turrou, who promptly sold his story to the newspapers, much to Hoover’s fury.

In May 1940, in the continuing absence of any formal liaison with the FBI, MI5 detained a cipher clerk working at the U.S. embassy in London and charged him with stealing copies of Winston Churchill’s private correspondence with President Roosevelt. The trial of Tyler Kent was held in secret, but the episode would have lasting political implications, especially for U.S. Ambassador to the U.K. Joe Kennedy, who had withdrawn his subordinate’s immunity.

Later in World War II, MI5 attempted to send a German double agent, Dusko Popov, codenamed TRICYCLE, to the United States to fulfill an assignment for German intelligence, but Hoover was outraged by the louche behavior of the Yugoslav playboy and forbade his staff from assisting the self-confessed spy’s mission. Accordingly, Popov only partially completed his tasks and was forced to return to Lisbon where he was regarded with skepticism by his German controller.

Get your history fix in one place: sign up for the weekly TIME History newsletter

Understandably, Hoover refused to cooperate with any branch of British Intelligence after he discovered during this period that the Spanish and French embassies in Washington had been targeted for penetration, having previously received a solemn pledge that no such activities would be conducted in his jurisdiction. Even worse, he discovered that the British were secretly collaborating with his rival, General Bill Donovan, the Chief of the U.S. Office of Strategic Services, a precursor to today’s CIA.

In the postwar era, MI5 would come to rely on a cryptographic source, designated VENONA, that would become the gold standard for its Cold War counter-espionage. The VENONA material — intercepted Soviet cablegrams — in 1951 proved that Donald Maclean, then head of the Foreign Office’s American Department, was a long-term Soviet mole. The same material, almost 3,000 individual messages exchanged with Moscow starting in 1940, implicated hundreds of other spies. Other members of the network, such as MINISTER and BARON, remain tantalizingly unidentified.

Later, an MI5 operation to entice a Soviet diplomat into defecting was the catalyst for the Profumo scandal, which contributed to the end of Harold Macmillan’s administration in 1963. MI5 had attempted to recruit a well-connected society osteopath, Dr. Stephen Ward, as an access agent who was to honey-trap Soviet Assistant Naval Attaché Evgenni Ivanov — ignorant of Ward’s involvement in a call-girl racket that stretched across the Atlantic and would prompt an FBI inquiry, codenamed BOWTIE, that threatened to engulf the Kennedy family too.

MI5’s propensity for engaging in high-risk operations remains a constant thread throughout its history. By their nature, counter-intelligence and counter-terrorism entail controversial sources and methods which all have the potential for wrecking political careers. Over the course of the last century, MI5 and the FBI have embarrassed and annoyed each other — and worse —and their relationship has often been a tense one.

Nevertheless, a London posting is highly regarded within both the FBI and CIA. The current CIA Director, Gina Haspel, served as the station chief in London twice, and the new special agent in charge of the Counterintelligence Division for the FBI’s Washington Field Office served as the FBI’s link to British intelligence. The intelligence relationship between the two countries hasn’t always been smooth, but it has been — and remains — indispensable.

Rupert Allason writes about espionage under the pen name Nigel West. He is the author of MI5: British Security Service Operations 1909-1945, available now from Pen & Sword, distributed in USA/CAN by Casemate.