Colonel Lord Sempill: Did he gave Japan the Secrets of Pearl Harbor?
Colonel Lord Sempill: The Peer Who Gave Japan the Secrets of Pearl HarborHe was a celebrated air pioneer, a Scottish peer, and a close friend of the Royal Family. He was also the man who taught Japan how to build an aircraft carrier—and then kept supplying secrets until the bombs fell on Pearl Harbor.
When British Intelligence finally caught him red-handed in December 1941, just days after Japan invaded Malaya, they faced an impossible choice. Prosecute him for treason, or protect the reputation of the British establishment?
They chose to protect the establishment.
Who Was Lord Sempill?
William Francis Forbes-Sempill, 19th Lord Sempill (24 September 1893 – 30 December 1965), was a Scottish peer, record-breaking aviator, and aeronautical engineer .
Before the war, his credentials were impeccable:
He was a genuine hero of early aviation—a man who broke records, pushed boundaries, and earned the trust of the British military establishment.
And he was also a spy for the Empire of Japan.
The 1921 Mission: Teaching Japan to Fly
In 1921, Sempill led an official British Aviation Mission (BAM) to Japan. His task was to help organize the air service of the Imperial Japanese Navy .
The mission was legal. It was even approved by the British government, which saw Japan as a friendly power at the time.
But what Sempill did next crossed every line.
Over the next several years, he continued to supply the Japanese with classified information about British aircraft technology—long after his official mission ended .
By 1924, MI5 was monitoring his relationship with the Japanese naval attaché in London. They turned up prima facie evidence of breaches of the Official Secrets Act .
Yet when Sempill was confronted on 4 May 1926, he faced no prosecution. The Director of Public Prosecutions advised against it .
Why? The evidence was there. But charging a Scottish peer and war hero with espionage would have been scandalous.
So Sempill walked free.
The Second Act: 1939–1941
When war broke out in Europe in 1939, Sempill was given a position in the Department of Air Materiel at the Admiralty .
This gave him access to highly sensitive information about the latest Fleet Air Arm aircraft—exactly the kind of intelligence Japan desperately needed as it prepared for war in the Pacific.
By June 1941, MI5 had intercepted messages between London and Mitsubishi headquarters in Tokyo. One cable was damning:
"In light of the use made of Lord Sempill by our military and naval attaches in London, these payments should continue" .
The Japanese were paying him. And MI5 knew it.
Yet again, the Attorney General advised against prosecution. On 5 September 1941, Sempill was given "a strict private warning" and allowed to continue his work .
The Arrest of Makihara: Sempill Intervenes
On 2 August 1940, Special Branch arrested Makihara Satoru, the head of Mitsubishi's London office, on suspicion of espionage .
What did Lord Sempill do?
He telephoned the police station. Then he went there in person to vouch for Makihara's character .
The Japanese businessman was released days later due to "insufficient evidence" .
Who else in Britain would personally intervene to free a suspected Japanese spy during wartime?
The Churchill Connection: Leaks at the Highest Level
In August 1941, Winston Churchill and President Franklin D. Roosevelt held a secret meeting in Newfoundland aboard HMS Prince of Wales to discuss the Japanese threat .
Soon after, Bletchley Park codebreakers deciphered Japanese communications containing transcripts of the conference notes .
An alarmed Churchill called them "pretty accurate stuff."
Three months later, more notes—this time from Churchill's personal agenda—were intercepted being sent from the Japanese Embassy in London to Tokyo.
Privately, Churchill concluded with Foreign Secretary Anthony Eden that only two men could be the source of such leaks: Commander McGrath (Sempill's aide) or Lord Sempill himself .
The Cover-Up: "Clear Him Out While Time Remains"
On 9 October 1941, Churchill signed a note:
The Admiralty told Sempill to resign. But when Sempill protested, Churchill backtracked.
The Prime Minister told the Admiralty: "I had not contemplated Lord Sempill being required to resign his commission, but only to be employed elsewhere" .
Sempill was offered a post in northern Scotland—effectively exiled, but not charged, not arrested, and not publicly disgraced .
Caught Red-Handed: December 1941
On 13 December 1941, six days after the attack on Pearl Harbor, MI5 raided Sempill's office .
They found secret documents that he should have returned to the Admiralty over three weeks earlier.
Two days later, Sempill was discovered making phone calls to the Japanese Embassy .
Britain and Japan were now at war. The Japanese had already invaded Malaya on 8 December 1941. Singapore was under threat.
And Lord Sempill was still talking to the enemy.
The Unanswered Question: Why No Prosecution?
Despite overwhelming evidence of wartime treason—which carried the death penalty under the Treachery Act 1940—Sempill was never arrested, never charged, and never publicly named .
Why?
According to intelligence historian Richard Aldrich, quoted in the book The Secret State:
"This is a classic case of Churchill protecting himself. If Sempill had been revealed as a spy, it would have been politically calamitous for Churchill at a low point in the war" .
Sempill was a peer. He had family links to the Royal Family . Prosecuting him would have exposed:
British intelligence failures spanning two decades
The betrayal of American trust (Japan used Sempill's intelligence at Pearl Harbor)
The embarrassing reality that a British lord helped arm the enemy
So Sempill was quietly retired. He lived another 24 years, dying in 1965 at age 72, never facing justice .
The Contrast: Sempill vs. Rutland
Sempill was not alone. He worked alongside another British officer: Captain Rutland (the basis for the 2012 documentary The Fall of Singapore: The Great Betrayal) .
The documentary's expert summed up the double standard:
Rutland went to jail and later killed himself. Sempill retired to his Scottish estate.
Class, it seems, protected him where the law could not.
The Malaya Connection: Why This Matters to 1942MALAYA
The intelligence Sempill passed to Japan directly contributed to the fall of Malaya and Singapore:
| Sempill's Contribution | Impact on Malaya |
|---|---|
| Aircraft carrier technology | Enabled Japanese naval air power in the Pacific |
| Fleet Air Arm secrets | Informed Japanese tactics against British forces |
| Churchill's conference notes | Revealed Allied strategy and weaknesses |
| Technical intelligence on RAF aircraft | Helped Japanese pilots understand their enemy |
The Japanese knew exactly what they were facing because a British peer had told them.
When General Yamashita's 25th Army swept down the Malay Peninsula in December 1941, they did so with intelligence that Sempill had helped provide.
The Disappearing Files
Sempill's case remains clouded in mystery. According to The Secret State:
"Most of the intelligence files on Sempill's activities during the 1930s and 1940s have mysteriously disappeared" .
What remains shows that Japan had a highly organized spy operation in Britain with at least five British citizens providing information to Tokyo .
Sempill was the highest-ranking among them. But he was far from the only one.
The Academic Debate: Spy or Scapegoat?
A 2023 peer-reviewed article in Studies in Conflict & Terrorism offers a contrarian view. The author argues that while Sempill's conduct gave "reasonable grounds for investigation," MI5 turned up "no evidence of treachery" .
The prevailing view of Sempill as a "traitor of Pearl Harbor" may, according to this fresh analysis, be a "canard" that has distorted our understanding of the period .
But the evidence is difficult to dismiss:
Payments from Japanese intelligence
Classified documents in his possession
Phone calls to the Japanese Embassy after war was declared
Churchill's own conclusion that Sempill was the likely source of catastrophic leaks
At minimum, Sempill was grossly compromised. At worst, he was one of the most damaging spies of the 20th century.
Legacy: A Traitor Who Was Never Named
Sempill died in 1965. His daughter, Ann Forbes-Sempill, succeeded him as the 20th Lady Sempill and took his seat in the House of Lords .
The full extent of his betrayal was not widely known until the release of MI5 files in the late 1990s .
Today, historians still debate whether he was a traitor, a fool, or a man caught between loyalties.
But one fact remains undisputed: Lord Sempill gave Japan the tools to attack. And Britain let him walk free.
For the soldiers who died defending Malaya and Singapore in 1941–1942, that was the greatest betrayal of all.
Further Reading on 1942MALAYA
Top 5 Resources
Taylor & Francis Online (Studies in Conflict & Terrorism) – 2023 peer-reviewed academic article reexamining the Sempill case and questioning the "traitor of Pearl Harbor" label.
Wikipedia – William Forbes-Sempill, 19th Lord Sempill – Comprehensive biographical entry detailing his aviation career, espionage activities, and the Churchill cover-up.
"The Secret State: A History of Intelligence and Espionage" – Book excerpt detailing Churchill's protection of Sempill and the "disappeared" intelligence files.
IMDb – "The Fall of Singapore: The Great Betrayal" (2012) – Documentary contrasting Sempill's fate with that of Captain Rutland, who went to prison while the peer walked free.
UK National Archives (via NAS Singapore) – Records of SOE operations in Malaya and Singapore, contextualizing the intelligence environment Sempill compromised.
What do you think? Was Lord Sempill a traitor who should have faced the firing squad, or a scapegoat for broader British intelligence failures? Share your thoughts in the comments below.
Support this site with Buy Me Coffee
Comments
Post a Comment