How much did Churchill know - The UK Perspective
Throughout World War II, British Prime Minister Winston Churchill and US President Franklin Delano Roosevelt (Roosevelt, Franklin D.) were portrayed as the closest of friends. In fact, Roosevelt had only met Churchill once briefly before the war and found him extremely rude. His closest advisors dismissed Churchill as a 'drunk and a windbag.' Although they appeared friendly in front of the cameras, Roosevelt never fully trusted Churchill and carefully distanced himself from Churchill's plans.
Roosevelt was right to be wary. From the moment Churchill returned to government with the outbreak of war in September 1939, he planned to defeat Germany by dragging America into the war. While Roosevelt was keen to help Britain, and eventually came to believe that war with Germany was inevitable, he was keen not to be dragged into Churchill's imperial adventure.
Throughout the first year of the war, Churchill bombarded Roosevelt with flattering messages. At the same time, he skillfully manipulated the intelligence he shared with Roosevelt to maneuver America to the brink of war. The Americans had cracked the Japanese diplomatic codes and, in January 1941, they gave the British the Purple and 'Red' decoding machines which allowed the British to read Japanese diplomatic traffic. In return, they expected to be given the German Enigma code machine, so they could break the German codes. The British refused to hand one over. It was not until after the attack on Pearl Harbor that the Americans learned the British had broken the Japanese navy code JN-25.
The British had listening posts in the Far East, with headquarters in Singapore. From 1939, they had been on a war footing and priority was given to intercepting enemy messages and decoding them. Churchill insisted on seeing all JN-25 messages personally.
While America was still at peace, code-breaking was not given priority. Its western-most listening station was in Seattle. Some of the crucial intercepts indicating that the Japanese intended to attack Pearl Harbor were not decoded until after 1945, and due to security considerations, President Roosevelt was often not privy to raw intelligence. There were US liaison officers at the British decoding center in Singapore, but they were not allowed to see raw intelligence and did not even know that the British had broken JN-25. British and Australian intelligence officers sent all their decrypts back to London, assuming that intelligence concerning an attack on Pearl Harbor would be forwarded to the Americans. It was not.
Although Churchill knew of the Japanese intentions, he deliberately misled Roosevelt by exaggerating the British strength in Singapore. He gave the impression that this was where the first attack would come - or, at the very least, that the Japanese would split their fleet and attack British and American forces simultaneously. However, he knew this was not the case. A report had already been drawn up showing that Singapore could not be defended. It would take at least 90 days for a fleet to reach Singapore from Britain, and besides, all available vessels were needed to protect the Atlantic convoys. In his mind, Churchill had already abandoned Singapore. And the Japanese knew it. The defense report had been on its way out to Singapore when it had been captured by a German ship and forwarded to Tokyo. The Japanese knew that it was not necessary to make a first strike against Singapore. It would fall anyway. Churchill knew that too, but he maintained the pretense that the first attack would come there, to the extent of pouring in British and Australian troops who would end up in Japanese prisoner of war camps with hardly a shot being fired. Not even the Australian prime minister was informed of the deception.
On November 19, 1941, the Foreign Ministry in Tokyo sent out a 'Purple' message to its embassies and consulates around the world. It instructed them to listen to Japanese news bulletins. If they ended with a weather report saying 'east wind rain' the attack would be on the US. 'North wind cloudy' would mean an attack on Russia, and 'west wind clear' would herald an attack on the British, with an invasion of Thailand or Malaya, or an attack on the Dutch East Indies. Both the British listening station in Melbourne and the American station in Seattle intercepted this and reported it to London and Washington respectively. While the Japanese diplomatic traffic still talked of negotiation, JN-25 traffic intercepted by the British talked of 'opening hostilities.' From November 21, it was clear that an attack was being set in motion and a large Japanese fleet was being assembled. Meanwhile, Japanese merchant ships were sailing home.
On November 25, the British intelligence headquarters in Singapore decoded a JN-25 message from Admiral Yamamoto (Yamamoto, Isoroku) saying: 'The Task Force will move out of Hitokappu Wan [Tankan Bay in the Kuriles] on the morning of November 26 and advance to the standing-by position on the afternoon of December 4 and speedily complete refueling.'
As negotiations with the Americans were continuing, it was unlikely that the Japanese would have sent a fleet towards Singapore or Manila as they would almost certainly be spotted by merchant shipping in the busy southern waters and the reconnaissance planes which patrolled that area. So Pearl Harbor was the likely objective. It is not clear whether this information was conveyed from Churchill to Roosevelt as those documents are still classified. However, there was a considerable hardening of Roosevelt's negotiating position with the Japanese the following day.
On 2 December, the Singapore station decoded a message from Yamamoto saying: 'Climb Mount Niitaka 1208.' This was the signal for the attack to go ahead and gave the date of the attack - 8 December, in Tokyo, 7 December in Pearl Harbor. As each day passed and no Japanese fleet was spotted heading for Singapore or the Philippines, intelligence analysts in Singapore became all the more convinced that the attack would be on Pearl Harbor. They informed London, assuming that the warning would be forwarded to the Americans. It wasn't.
On 4 December, Japanese news bulletins ended with a weather forecast predicting 'east wind rain' -the code for an attack on America. This was broadcast three times before anyone in American intelligence realized that the attack was imminent. But without the JN-25 intelligence, it was still not clear where the attack would come.
Staff shortages in the American decoding rooms meant that Churchill knew the contents of Tokyo's final communiqué to Washington before Roosevelt did. He had time to invite the American ambassador John Winant and Roosevelt's special envoy to Britain, Averell Harriman, down to Chequers for dinner. At 9pm, he asked his butler to bring a portable radio into the dining room so they could listen to the evening news. It announced that Pearl Harbor was being attacked.
'No American will think it wrong of me if I proclaim that to have the United States on our side was to be the greatest joy,' Churchill said. 'Once again in our long island history we would emerge+ safe and victorious. Being saturated and satiated with emotion and sensation, I went to bed and slept the sleep of the saved and thankful.'
However, the British intelligence analysts at the Singapore station were puzzled. If the Americans had received all the JN-25 intelligence reports they had sent back to London, how could they had been taken by surprise? If they had received those reports, they would have had more than enough warning to prepare a trap, or perhaps even to prevent the attack on Pearl Harbor occurring at all.