'Yanagi' was the Japanese programme, known to the Germans as 'Kirschblute', in which Japanese submarines travelled to Europe to collect high-value military cargoes such as weapon blueprints, weapon components and special materials unavailable in the Far East (June 1942/May 1945).

After the Japanese 'Ai' attack on Pearl Harbor on 7 December 1941 and Germany’s declaration of war on the USA on 11 December, the Tripartite Pact of 27 September 1940 was amended to provide for an exchange of strategic materials and manufactured goods between Germany, Italy and Japan. The first exchanges were made by blockade-running ships but then, as the tide of the naval war started to turn against the Axis powers submarines came to be seen as the better means of transport. From the beginning, though, it was appreciated that the cargo capacity of the submarines used for this task would of necessity be considerably smaller than that of any surface ship.

As early as 27 March 1942, the German naval high command requested that the Imperial Japanese navy undertake offensive operations against Allied convoys in the Indian Ocean to help relieve some of the pressure on the Kriegsmarine, and on 8 April the Japanese agreed to dispatch submarines to operate off the coast of East Africa. Shortly after this, the boats of the 1^st Division of the 8^th Submarine Squadron were withdrawn from Kwajalein in the Marshall islands group and sent to operate from Penang island off the west coast of Japanese-occupied Malaya.

Commander Shinobu Endo’s large ocean-going I-30, characterised by surfaced and submerged displacements of 2,584 and 3,654 tons respectively and the prodigious range of 16,100 miles (25910 km), was one of the boats assigned to Captain Noboru Ishizaki’s 8^th Submarine Squadron, in which it joined I-10, I-16, I-18 and I-20 and their support ships in the 'Ko' Detachment. On 22 April I-30 departed Penang and a week later assisted in the detachment’s successful attack on British shipping at Diégo Suarez on the northern tip of the Vichy French island of Madagascar, which the British were attacking on 'Ironclad'. The Japanese boat damaged the battleship Ramillies and sank a tanker. After the attack, I-30 patrolled in the area to the east of Madagascar for a time before being ordered on the first 'Yanagi' mission undertaken by a submarine. For this task the boat was placed directly under the command of Vice Admiral Teruhisa Komatsu’s 6^th Fleet.

On 2 August I-30 entered the Bay of Biscay and, off Cape Ortegal in north-western Spain was met by eight Luftwaffe Junkers Ju 88A warplanes to provide air cover. Three days later, I-30 was met by a flotilla of minesweepers and escorted to Lorient which was, at that time, the largest of the five German U-boat bases on the coast of occupied France.

I-30 was the first Japanese submarine to arrive in Europe, and was carrying a cargo of 3,307 lb (1500 kg) of mica, 1,455 lb (660 kg) of shellac and a complete set of engineering drawings of the Japanese Type 91 air-launched torpedo.

German submarine experts took the opportunity to cast a critical eye over the Japanese boat, and came to the conclusion that its noise levels were notably high by German standards and therefore that the boat would be comparatively easy to locate and track by hydrophone or sonar. The Germans also decided that the Japanese boat also had a large radar cross section, and this too would make it a simple target for location by Allied radar-equipped aircraft. The Germans therefore fitted a Metox 'Biscay Cross' passive radar detector to the bridge of the Japanese boat, and also replaced its 25-mm Type 96 anti-aircraft cannon and replaced them with a quadruple mount of four faster-firing 20-mm cannon. Repairs were also made to I-30's Yokosuka E14Y1 'Glen' scouting floatplane.

The Japanese visit ended on 22 August as I-30 departed for its passage to Japan. The boat’s cargo now included a complete Würzburg air defence ground radar with blueprints, as well as examples of German torpedoes, bombs, fire-control systems and, perhaps most important of all, industrial diamonds valued at one million yen and 50 Enigma top-secret coding machines.

One month later, I-30 rounded the Cape of Good Hope and entered the Indian Ocean and early in the morning of 8 October arrived back at Penang. Rear Admiral Zenshiro Hoshina, chief of the Japanese navy’s logistics branch, was on hand and asked Endo for 10 of the Enigma machines. Two days later the boat departed Penang and headed south for Singapore via the Strait of Malacca.

During the night of 13 October the boat reached Singapore, but it was not until the following morning that the submarine was able to make its way into the port. Vice Admiral Denshichi Okawachi, commander of the 1^st Southern Expeditionary Fleet, and staff of the 10^th Special Base Unit were on hand to greet the boat, and on the same day, the submarine’s navigator requested and received maps of the areas round Singapore which had been swept of mines. Anxious to get home, Endo departed Singapore for Kure that afternoon, but only 3 miles (4.8 km) to the west of Keppel Harbour the boat hit a mine and was crippled: Endo and most of his crew were rescued before the boat sank.

Divers were immediately dispatched to recover I-30's cargo, but discovered that the Würzburg radar had been destroyed in the blast and its drawings obliterated by sea water. In addition, the remaining Enigma machines were lost, though this fact was not disclosed to the Germans for four months.

Although I-30's pioneering 'Yanagi' voyage had ended somewhat ignominiously, much had been learned. On 31 March 1943, the Japanese ambassador to Germany reported to Tokyo that Generalfeldmarschall Erich von Manstein had suggested that since so many surface blockade-runners were being sunk that large, older U-boats should be converted to carry war matériel between Europe and the Far East. The Japanese ambassador recommended that von Manstein’s suggestion be implemented as rapidly as possible, but the cable was sent in the Japanese 'Purple' diplomatic code, which was intercepted and decoded by the Allies.

On 1 June, the 2,525/3,583-ton I-8 departed Kure with the 2,919/4,150-ton I-10 and submarine tender Hie Maru. Commander Shinji Uchino had just been given his orders to proceed to Lorient. The cargo carried by I-8 included two Type 95 oxygen-propelled torpedoes, drawings of an automatic trim system, Type 95 submarine torpedo tubes and a new naval reconnaissance aeroplane. Accompanying Uchino was Lieutenant Commander Sadatoshi Norita and a 48-man spare crew, and this crew was to be trained by the Germans in the Baltic Sea before taking over U-1224, a 'Type IXC/40' U-boat. Also on board I-8 were four translators and code clerks, a medical officer and an expert on torpedo boat engines. Nine days later I-8 reached Singapore, where it loaded an additional cargo of quinine, tin and raw rubber before heading for Penang. I-8 then departed alone on its westward passage at a timer late in June. On 21 July the boat entered the Atlantic and was greeted by fierce storms that battered it for 10 days.

On 24 July the boat received its first radio signal from the Germans, who warned of the probable presence of Allied radar-equipped air patrols. Five days later the boat received a second signal instructing it to head for Brest rather than Lorient. I-8 crossed the equator on 2 August, and 18 days later made rendezvous with Kapitänleutnant Albrecht Achilles’s U-161. On the following day the Japanese boat took aboard a Leutnant Jahn and two petty officer radiomen, who installed a FuMB 1 Metox 600A radar detector on the boat’s bridge. On 29 August the boat entered the Bay of Biscay, and was covered by Ju 88 aircraft as it travelled to the north-east to reach Brest two days later.

After a little more than one the Japanese boat departed on 5 October with a cargo including machine guns, bomb sights, a Daimler-Benz torpedo boat engine, naval chronometers, radar and sonar equipment, anti-aircraft gun sights, electric torpedoes and penicillin. The passengers were the Japanese naval attachés to Germany and France, three German naval officers, an army officer and four radar and hydrophone technicians.

After crossing the equator, Uchino sent a position report to the Germans, but the signal was intercepted by the Allies and on the following day I-8 was attacked by an anti-submarine aeroplane but managed to crash-dive and escape. On 13 November the boat passed to the south of Cape Town. On the same day I-34, which had just departed for France, was torpedoed by the British submarine Taurus to the south of Penang, so becoming the first Japanese submarine to be sunk by a British submarine. In view of the danger now posed by Allied boats, Uchino was ordered to head directly for Singapore, which I-8 reached on 5 December.

I-8 anchored near Commander Takakazu Kinashi’s I-29, which had just arrived from Kure and was about to depart for France. The two commanders met and Uchino warned Kinashi of the many radar-equipped Allied patrol aircraft he had encountered. He also praised the Metox radar detector which he had received from U-161.

After a short rest at Singapore, I-8 set off once more and reached Kure on 21 December at the conclusion of a voyage of some 30,000 miles (48280 km).

Meanwhile, I-29 departed Singapore for France under the command of Kinashi, who as captain of I-19 had become Japan’s leading submarine 'ace', credited with sinking the US fleet carrier Wasp off Guadalcanal on 15 September 1942, and damaging the battleship North Carolina and the destroyer O’Brien, of which the eventually broke in two and sank.

While Kinashi was new to the 'Yanagi' mission, his boat and much of his crew were not. On 5 April 1943, I-29 had departed Penang on a secret operation with 11 tons of cargo, including one Type 89 torpedo, two Type 2 air-launched torpedoes and two tons of gold for the Japanese embassy in Berlin. I-29 was also carrying drawings and blueprints of a 'Type A' midget submarine and the fleet carrier Akagi, which the Germans wished to study as they constructed their own carrier, Graf Zeppelin. On 25 April, about 450 miles 725 km) to the south-east of Madagascar, I-29 arrived at the point at which the boat was to rendezvous with Kapitänleutnant Werner Musenberg’s U-180. Musenberg’s boat was the first to make the eastward journey to meet a Japanese boat, and had departed Kiel on 9 February carrying blueprints for the 'Type IXC/40' U-boat, a sample of a German hollow-charge warhead, a quinine sample for future Japanese shipments, gun barrels and ammunition, three cases of sonar decoys, and documents and mail for the German embassy in Tokyo. The U-boat also carried Subhas Chandra Bose, the head of the anti-British Indian National Army, and his Moslem aide-de-camp, Major Habib Hassan The two boats met, as planned on 26 April.

After the transfer of a German officer and a signalman, the two submarines continued together on a north-easterly course waiting for the seas to calm enough for the exchange of cargoes. On the following day, Bose and his group transferred from the German to the Japanese boat in a rubber raft, and two Japanese officers transferred to the U-boat. The 11 tons of cargo were then floated across on three inflatable rafts while both submarines had their torpedo hatches open. After the passenger and cargo exchanges had been completed, I-29 headed to the east and U-180 to the south to turn the Cape of Good Hope for the Atlantic and its base at Bordeaux in occupied France.

This experience was very useful for Kinashi when I-29 departed for France on 16 December. In addition to its crew, the boat carried a cargo of rubber, tungsten, tin, zinc, quinine, opium and coffee, as well as 16 Japanese naval officers, specialists and engineers, most of whom had been scheduled to depart Penang with the ill-fated I-34, which had been sunk before reaching Penang. After a week at sea, I-29 was refuelled from the small German supply ship Bogota in a process which lasted almost six hours, and then proceeded on its way. On 8 January 1944 the submarine passed south of Madagascar.

Early in February Kinashi received a signal from Germany to rendezvous with a U-boat which would supply him with a newer radar detector. On 12 February I-29 met Oberleutnant Hans-Werner Offermann’s U-518 to the south-west of the Azores islands group, and took on board three technicians who removed the Metox radar detector transferred from Uchino’s boat and installed a new FuMB 7 Naxos detector on the bridge.

On 4 March, while running on the surface off Cape Finisterre. During the evening, an RAF patrol aeroplane carrying Leigh Light 24-in (610-mm) searchlight suddenly illuminated the water around I-29. Reacting with the speed and efficiency gained from long experience, Kinashi and his crew managed to crash-dive their submarine and escaped unharmed. Five days later, I-29 entered the Bay of Biscay, but had arrived here before his escort and had to spend the night on the bottom. On the next morning I-29 rendezvoused with five Junkers Ju 88C heavy fighters, and in the afternoon with two German destroyers and two torpedo boats, which escorted the Japanese submarine toward Lorient.

Allied code breakers had intercepted transmissions which indicated I-29's likely position and schedule, however, and had dispatched two specially equipped de Havilland Mosquito anti-ship fighters each armed with one 57-mm cannon, together with four other Mosquito aircraft of No. 248 Squadron to attack the submarine and its escorts. The British found the ships off northern Spain under escort of eight Ju 88C fighters. The Mosquito fighters tried to draw the German aircraft away so that the cannon-armed machines could attack the submarine and its escorts. The British succeeded in downing one Ju 88, but I-29 was undamaged. Later on the same day the boat and its escorts were attacked by more than 10 Allied aircraft, including Bristol Beaufighter heavy fighters and Consolidated B-24 Liberator patrol bombers.

On 11 March I-29 reached Lorient and anchored next to Kapitänleutnant Max Wintermeyer’s U-190 before being berthed in the massive Keroman concrete pen structure, home of two U-boat flotillas. In Lorient the Germans replaced the Japanese boat’s four obsolescent 25-mm anti-aircraft cannon with heavier 37-mm anti-aircraft cannon and one 20-mm cannon. In addition, a HWK 509A-1 rocket motor of the type used in the Messerschmitt Me 163 Komet rocket interceptor and a Junkers Jumo 004B axial-flow turbojet engine as used in the Messerschmitt Me 262 jet fighter were loaded, together with drawings of the Isotta-Fraschini torpedo boat engine, the fuselage of a V-1 flying bomb, acoustic mines, bauxite ore and mercury/radium amalgam. Some evidence also suggests that I-29 carried a quantity of U-235 uranium oxide, which after refining could have been used in an atomic bomb. Two officers were entrusted with blueprints of the Me 163 and Me 262 warplanes, plans for rocket launch accelerators, and plans for a glider bomb and radar equipment. Finally, 20 more Enigma coding machines were loaded. So loaded, I-29 departed Lorient on 16 April.

On June 11 I-29 passed I-52 in the South Atlantic: the boats did not communicate, but Kinashi intercepted some German radio traffic addressed to I-52. Some 18 days later, I-29 entered the Indian Ocean and on 13 July made rendezvous with its air escort. On the following day the boat traversed the Strait of Malacca and reached Singapore, where the passengers disembarked with their plans and documents before continuing to Japan by air. Most of the scientific cargo, however, remained aboard.

Anxious as to the exact whereabouts of the Japanese boat, Allied code breakers were greatly relieved when they intercepted a signal that indicated its arrival in Singapore, but intelligence personnel when another signal intercept gave the details of the submarine’s strategically important cargo. Aware of the frightening potential of what was being carried in I-29's hold, Allied intelligence began working around the clock to devise a way to intercept the submarine before it could reach Japan.

On 20 July Kinashi transmitted his proposed route for the last leg of the trip. The US Navy’s Fleet Radio Unit, Pacific (FRUPAC) intercepted and deciphered the message and alerted Vice Admiral Charles A. Lockwood, commander of the Pacific Fleet’s submarine force, of I-29's planned route, schedule and cargo. Lockwood then sent a top-secret signal to Commander W.D. Wilkins, commander of a wolfpack which included his own Tilefish as well as Rock and Sawfish. Wilkins was tasked to ensure that I-29 was intercepted and sunk.

Unaware that his plan had been discovered, Kinashi departed Singapore on the morning of 22 July. Three days later, he reported sighting a surfaced US submarine, and during the afternoon of the following day, as I-29 was itself running on the surface through the western entrance of the Balintang Channel in the Luzon Strait, a lookout on Commander Alan B. Banister’s Sawfish sighted the Japanese submarine. Banister fired four torpedoes at I-29: three of these hit their target, and the Japanese submarine exploded and sank almost immediately.

Three of the Japanese crew were blown clear of the boat, and one of these managed to swim ashore to a small Philippine island and report I-29's fate. The loss of the aircraft engines slowed the Japanese jet programme, but their blueprints, flown to Tokyo, arrived safely. The plans were used in the development of the Nakajima Kikka and Mitsubishi J8M Shusui based on the Me 262 and Me 163 respectively.

All Japanese hopes for technological transfers from Germany now lay with Commander Kameo Uno’s 2,564/3,644-ton I-52, which had departed Kure on 10 March 1944 carrying strategic metals including molybdenum and tungsten, and 146 bars of gold packed in 49 metal boxes, as well as opium and some caffeine. I-52 also carried 14 passengers including engineers and technicians who were to study German weapon systems.

On reaching Singapore, the boat additionally loaded tin, rubber and quinine, and departed once more on 23 April for Lorient via the Sunda Strait and the Indian Ocean. To avoid detection by Allied aircraft, Uno planned that his boat would travel submerged during the day and surface only at night to recharge the batteries.

After passing the Cape of Good Hope and entering the South Atlantic, on 15 May Uno sent his first message to Germany. By this time the Allies had broken the current German and Japanese codes, and intercepts revealed a number of useful things including the Japanese boat’s daily noon position reports. When I-52 entered the South Atlantic, therefore, the code breakers were able to report the boat’s position and predicted course to a US Navy anti-submarine warfare task force.

On 6 June the new Japanese naval attaché in Berlin informed Tokyo and I-52 of the Allies 'Overlord' landing in Normandy. He advised I-52 that it might have to divert to Norway and instructed Uno to rendezvous at sea with a U-boat on 22 June. He then informed Tokyo of where I-52 would be on this date, and the signal was intercepted and decrypted by the 'Ultra' system, allowing a US anti-submarine force operating off the Azores islands group to be primed.

On 16 I-52 signalled to the effect that it was off West Africa, and Captain A. B. Vosseller’s escort carrier Bogue, carrying 14 aircraft, was ordered to intercept and destroy I-52. After arriving in the area where the Japanese boat was to meet the U-boat, Vosseller ordered round-the-clock flights of Grumman TBF Avenger torpedo bombers to search for the two Axis submarines.

Uno’s I-52 rendezvoused with Kapitänleutnant Kurt Lange’s U-530 some 850 miles (1370 km) to the west of the Cape Verde islands group. Uno now received a German officer to help navigate the last leg of his journey, and this navigator was accompanied by two petty officers and the improved Naxos FuMB7 radar receiver. During the exchange, the radar detector fell into the sea, but a Japanese seaman retrieved it. About two hours after meeting I-52, U-530 submerged and headed for Trinidad, leaving the three Germans on board the Japanese submarine.

The day after this rendezvous, Uno believed that he could take advantage of a dark moonless night during stormy weather to travel on the surface and thereby reach the safety of a German-held port more quickly. At about 23.40 an TBF located I-52 on its radar, dropped flares to illuminate the area round the boat and then dropped two 354-lb (160-kg) depth bombs, which just missed the boat’s starboard side. Uno ordered a crash-dive, but while his boat had evaded the attack, it had now been located.

The TBF next dropped sonobuoys, and within minutes the aeroplane’s crew had picked up the sound of I-52's in their headsets. The pilot manoeuvred his aeroplane into position and dropped a new top-secret Mk 24 'Fido' acoustic-homing torpedo into the water, and a long wait, the US airmen heard a loud explosion.

On June 24, another Avenger arrived and dropped more sonobuoys, and then picked up the sounds of the damaged submarine’s propellers. At about 01.00 the pilot launched another 'Fido' torpedo toward the submarine, and then the US airmen heard the submarine breaking up underwater. The next day Janssen, one of Bogue's destroyer escorts, found a large oil slick at the site of the attack and salvaged more than a ton of raw rubber bales floating amid other debris.

At Lorient a German ship was standing by to escort I-52, and diplomats scheduled to return to Japan waited anxiously. With them at the dock were tons of secret documents, drawings and strategic cargo, which included T-5 acoustic torpedoes, a Junkers Jumo 213A engine of the type used in the Focke-Wulf Fw 190D fighter, radars, vacuum tubes, ball bearings, bomb sights, chemicals, steel alloy, optical glass and 992 lb (450 kg) of uranium oxide. The Germans also intended to equip I-52 with a Schnorchel underwater breathing system.

On 30 August the German navy declared I-52 as presumed sunk in the Bay of Biscay as of 25 July.

With the US forces now closing on the Japanese home islands, the Japanese navy needed to husband every available resource and decided no despatch no more submarines to Europe.