The US knew, in the second half of 1941, that Japan was preparing for war in the western Pacific and southeast Asia. Tokyo needed to secure material for its military operations in China – principally oil, tin, bauxite and rubber. But Washington was never aware of the final details of these plans.


US strategists knew, of course, that a Japanese offensive would chiefly target Dutch and British possessions in southeast Asia, because it was there that the raw materials required to fuel Japan’s imperial ambitions were located. They knew, also, that the US’s military presence in the Philippines would at some point come into the crosshairs. For some time, it had been clear that Japan was war-minded.

Emperor Hirohito’s expansionist regime had been beating the war drum in Asia since it had entered Manchuria in 1931, and had begun military operations elsewhere in China in 1937. The world had seen the alacrity with which it had forced a humiliated France to submit to its demands in Indochina in June 1940, and had watched Japan sign the Tripartite Pact on 27 September 1940 with the European fascist aggressor nations, Germany and Italy.

Above all, Washington knew about Japan’s plans for possible war – especially if the United States or the European colonial powers refused to peacefully allow it the raw materials to carry on its war in China – because American cryptographers had broken the Japanese diplomatic cypher.

But the United States never had any inkling, at any point before about 7.50am on 7 December 1941, that Tokyo’s plans for a general invasion of the region included a preventative and debilitating strike on the temporary home of the Pacific Fleet at Pearl Harbor. Subsequent attempts to suggest that President Franklin D Roosevelt – and by extension British Prime Minister Winston Churchill – knew of the impending attack and did nothing about it, in order to facilitate US entry into the war, haven’t a shred of historical evidence, and serve merely to paper over the deficiencies in American military planning that enabled the Japanese attack on Pearl Harbor to be so effective.

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This claim can be quickly dismissed. At the same time as the strike on Pearl Harbor, the Japanese launched a simultaneous attack on British Malaya – one that led to the fall of Singapore within 10 weeks. While Britain very much wanted the US in the war, this was to take on the Germans in Europe, not in the nightmare context of a fight on two fronts.

The Japanese assault on western colonial interests in southeast Asia was equally if not more calamitous for Britain than the United States, and welcomed by no one in London or Washington. For Britain, the need to fight in two theatres of combat was as unpleasant a surprise as the debilitating blow to the fleet at Pearl Harbor had been to US war planners.

A tale of complacency

The United States was aware of many elements of high-level Japanese political thinking as 1941 progressed, because it had managed to crack the country’s main diplomatic code – known as the ‘Purple cipher’ – in an operation codenamed ‘Magic’. The Japanese government and military used many different codes, but the Purple cipher was the only one fully mastered by US cryptographers. The naval cipher, JN25b, had only been partially unravelled by the time Japanese aircraft were making their initial dive-bombing runs against the Pacific Fleet.

Traffic between Tokyo and Japan’s embassy in Washington, then, could be read by the Americans, though diplomatic messages never carried explicit details of military plans or activities, usually giving high-level instructions and ‘lines-to-take’ for diplomats. Specifics of military plans were never entrusted to the radio, with or without encryption.

Japan's major triumph in the second half of 1941 was to keep the Pearl Harbor plan secret

All that Roosevelt and his secretary of state, Cordell Hull, knew of Japanese plans was what they could garner from the summary instructions General Hideki Tojo, the country’s recently appointed prime minister, was sending to his ambassador in Washington.

Tokyo had issued actual war orders on 5 November, and made a decision for war on 29 November, confirming it before the Emperor Hirohito on 1 December. These dates were known to Washington. Orders went to the Japanese armed forces to expect war on 8 December (an attack on Oahu at 08:00 hours on 7 December would fall at 03:30 hours on 8 December in Tokyo). However, this date was not promulgated to Japan’s embassy, so Washington was not aware.

Empire strikes: Japanese troops close in on Singapore in 1942. Their attack on British Malaya on 8 December 1941 was a nightmare scenario.
Empire strikes: Japanese troops close in on Singapore in 1942. Their attack on British Malaya on 8 December 1941 was a nightmare scenario. (Photo by ullstein bild/ullstein bild via Getty Images)

Japan’s major triumph in the second half of 1941 was to keep secret the plan to strike hard at Pearl Harbor, in the event that negotiations to secure its political ambitions in Asia were thwarted. The Japanese plan to emasculate US naval power in the Pacific, to allow it free rein in its seizure of the Philippines, Malaya and the Dutch East Indies, included a range of measures that have been common to all successful surprise attacks in history.

First, Japan carefully scouted the best route of attack: in this case, through the north Pacific, far from the normal shipping routes, which would enable the task force to avoid discovery by ships or aircraft as it circled towards Hawaii from the north. The route was reconnoitred by a civilian liner, which reported that it had sighted no other ships on its journey. During the actual operation, the Japanese attack fleet used climatic subterfuge to assist them, advancing beneath a cover of cloud and rain. They were not spotted.

Secondly, the armed forces exercised an iron discipline in terms of radio and signal traffic, to prevent plans being inadvertently leaked or tracked by an eavesdropper, while radio traffic around the Japanese home islands was boosted to make up for the absence of radio traffic from the fleet now making its way across the Pacific.

On top of this, Japanese carrier-borne air crews had practised relentlessly for months using mockups of the targets they expected to find anchored in Pearl Harbor, with pilots and crews of torpedo and dive bombers adding hundreds of hours to their flying logbooks for this single operation alone.

Japanese air crews had practised for months using mockups of the targets they expected to find at the base

Technical details were examined and problems ironed out – such as the depth to which torpedoes sank when dropped from aircraft into the shallow waters of a harbour (solved by adding wooden fins to the torpedoes), and concerns over the accuracy of the explosives dropped by the dive bombers. Every aspect of the Japanese operation was planned to the tiniest detail, and rehearsed accordingly, all without the Americans having any notion of what was to come. The plan was revealed to Japan’s Imperial Naval General Staff in August 1941 and confirmed – after much heated debate – on 3 November, only weeks before the attack was due to take place.

Radio silence: A Japanese signals unit in China, 1937. The ability to keep military communications secret was crucial to the attack on Pearl Harbor.
Radio silence: A Japanese signals unit in China, 1937. The ability to keep military communications secret was crucial to the attack on Pearl Harbor. (Photo by: Universal History Archive/Universal Images Group via Getty Images)

Sunday stand-down

The primary US failure was a cataclysmic underestimation of the enemy. It never entered American military consciousness that a massive ship-launched aerial bombardment could ever take place, at least without plenty of warning. And yet the Japanese attempted – and succeeded in achieving – the unthinkable. At the time of the attack, many of the standard countermeasures available to US forces on Hawaii were either switched off or not working. A British-made radar set, which had proven its worth during the battle of Britain the previous year, had been installed on Oahu to provide early warning of an air attack.

It worked, brilliantly, but the news that massed aircraft were heading towards the islands from the north was dismissed by the duty officer at Pearl Harbor, who was expecting a group of B-17 Flying Fortresses to arrive from California that same morning.

No regular reconnaissance sweep took off from the islands to search for hostile maritime interest towards the north – US searches from Oahu were confined to the southwestern sector – and nor was there a permanent combat air patrol flying high above the islands to detect intruders. Why should there be? The idea that 350 torpedo bombers, dive bombers and escort fighters would emerge from thin air and descend on a place 3,400 miles from Japan was absurd.

On the prize vessels of the Pacific Fleet, at weekend anchor on Battleship Row at Pearl Harbor, anti-aircraft ammunition was locked away. There was no one on anti-aircraft duty anyway, ships’ crews having been stood down for the sabbath. On land, only a handful of the army’s anti-aircraft guns had been supplied with ammunition, so slim were the chances of an air attack considered to be. Japanese intelligence-gathering on the island, meanwhile, had been assiduous, and Tokyo knew the US ships always returned to Pearl Harbor for the weekend, with Sunday regularly rostered as a stand-down day. In previous weeks, dry-run invasion exercises had been conducted by navy vessels on a Sunday morning – but “by some stroke”, one general testified at a Congress hearing, “we did not go out on 7 December. The fleet was in the harbour.”

The simple truth was that no one, on the American side at least, had any clue that Pearl Harbor was about to be attacked. The possibility had apparently never been war-gamed in the context of the developing Japanese threat in the western Pacific. There was no conspiracy. In Washington, there was instead merely a profound lack of planning and a naivety about what Japan’s military ambitions for its conquest of south-east Asia might entail. At the same time, on the Japanese side, a cunning and brilliantly executed military operation achieved precisely what its planners had intended: to prevent the US Pacific Fleet intervening in Tokyo’s imperial expansion push far to the southwest.


Robert Lyman is a writer and historian. His books on the Second World War include Japan's Last Bid for Victory: The Invasion of India, 1944 (Pen & Sword, 2011) and Under a Darkening Sky: The American Experience in Nazi Europe: 1939–1941 (Pegasus 2018)