Japan's World War II ambassador to Germany unwittingly supplied the Allies with detailed plans of German fortifications along the coast of France seven months before the invasion o Normandy.
After a two-week tour of German coastal defences in Normandy and Brittany in October and November of 1943. Japanese Ambassador Hiroshi Oshima radioed a 30-page message to Tokyo describing his trip. His briefings by the German High Command gave their plans for defending the coast and details of the German fortifications and weapons along the coast.
Oshima's message and a follow-up cable in even greater detail from his military aide were both decoded by the United States and are included in the thousands of pages of broken Japanese cables just turned over to the National Archives by the supersecret National Security Agency.
At about the same time the United States was reading descriptions of Germany's coastal defenses, it was also learning from many of the same cables about the supersecret German V2 rocket weapon. Once again, messages from Ambassador Oshima to Tokyo supplied some of the first clues.
"Germany's retaliation against England will come by rocket," Oshima told Tokyo in September 1943. Oshima then went on to place the weapons at Peenemunde and Friedrichshafen, the first time the Allies had any idea of where to look for the whispered-about rockets.
Oshima then advised Tokyo: "Please keep the names of these places utterly and absolutely secret."
On Nov. 10, 1943, just after he returned from his tour of Brittany and Normandy, Oshima told Tokyo that the Germans had deployed 31 divisions along the French coast and were maintaining another 15 "mobile" divisions inland to respond to an invasion wherever it came. Oshima said 1.4 million Germans were ready to defend France.
His numbers made the Allies immediately revise upward their estimate of German forces. The 46 divisions Oshima reported were five more than the Allies suspected were along the French coast at the time Oshima's wire was intercepted and decoded.
Far more important than the numbers were Oshima's descriptions of the fortifications, including the exact number of Tiger and Panther tanks in reserve behind them, an invaluable piece of intelligence for the Allies.
Oshima said the keystone of the German defenses was the orchestration of their gun batteries to fire in unison at a single target, before switching, again in unison, to another target.
"All the guns can be concentrated to fire on one object at the same time," Oshima said, "whether at sea or on land."
The Japanese diplomat went on to blueprint the way German antitank ditches were arranged, where mines had been placed and antitank guns located. He pointed out in meticulous detail how gun emplacements were arranged, which were camouflaged and which otherwise concealed.
Oshima said he was most impressed by the automotic grenade launchers and flame throwers placed in the German fortifications. The grenade machine could "throw" 120 grenades a minute and the flame thrower could be fired remotely from a bunker 100 feet away.
"The antiaircraft and antitank guns can also be fired on warships at sea and the fortress guns can be fired at tanks on land," the diplomat said. "This feature was greatly impressed on me by a night firing pratice La Baule in which all the guns fired at sea together."
In his folow-on message, Oshima's military aide (a Lt. Col. Nishi) described which guns turrets along the coast were armored and which were screeened from atack by barbed wire.
Oshima's second message about Germany's rocket weapon had this to say: "Talk in Germany centers more and more on a plan to use long-range rocket guns to shell London.The new rocket guns will be put into use by the middle of December at the latest and the effect of the weapons as a means of military retaliation will be equaled only by the political reverberations."
In a third message to Tokyo, Oshima said: "The Germans have demonstrated that very accurate hits can be scored [with his rocket] from a distance of 400 kilometers. The explosive power of the shell is so terrific that it will be effective against London itself not to mention the suburban factory towns."
The first of more than 1,000 V2s was launched from Peenemunde against London early in September 1944. Historical accounts of the presence of the weapons traced their discovery to British aerial reconnaissance.The Japanese diplomatic cables are the first signs that the British knew where to look almost a year before the V2 raids began.