'Cherry Blossoms at Night' was a Japanese unrealised plan to launch a biological warfare campaign against the USA’s civilian population of southern California during the final months of World War II (1945).

Commanded by Lieutenant General Shiro Ishii, Unit 731 (otherwise Detachment 731, 731^st Regiment, Manshu Detachment 731, Kamo Detachment, Ishii Unit Ishii Detachment and Ishii Company) had been established in 1935 by the Japanese military in Manchukuo specifically for research biological and chemical warfare, largely by experimentation on persons of all ages. During the 2^nd Sino-Japanese War (1937/45) and later in World War II, the Japanese had succeeded in making bubonic plague, cholera, smallpox, botulism, anthrax and several other diseases the payload of bombs which were deployed against Chinese troops and civilians. It is believed that the death toll from these diseases was between 400,000 and 580,000.

In the first months after its 'Ai' attack on Pearl Harbor, which caused the USA to enter World War II, Japan planned to use biological weapons against US forces. During their 'M' (ii) campaign to take the Philippine islands group, in March 1942 the Japanese gave consideration to the use of 200 lb (90 kg) of plague-carrying fleas in each of 10 attacks, but the US forces surrendered at Bataan before the plan was implemented. Around November 1944, in 'Fu' (ii), Japan launched a total of 9,300 incendiary and anti-personnel bombs carried by balloons which were designed to rise to 33,800 ft (10000 m), be swept to the east across the North Pacific by the jetstream to the continental USA, and then fall to release their payloads: these killed six US civilians near Bly, Oregon, crashed into a farm in Medford, Oregon, and caused a short circuit in the power lines supplying electricity for the nuclear reactor cooling pumps in the Manhattan Project’s production facility at the Hanford Site in Washington. During the US 'Detachment' campaign for Iwo Jima, another biological attack was considered against the invading US forces: two pathogen-carrying gliders were to be towed over the battle and released, but the gliders that were to be launched from the Japanese home islands to Pingfan airfield in Manchukuo, where the payloads were to be fitted, as the staging point for the attack did not reach their destination.

Following an inspection of Rabaul in August 1943, Captain Chikao Yamamoto and Commander Yasuo Fujimori conceived the idea of using the sen toku (secret submarine attack) capability for an attempt to destroy the locks of the Panama Canal nd thus cut US supply lines to the Pacific Ocean and hamper the transfer of US ships. Intelligence gathering on the proposed target began later in that same year.

The Japanese were well aware that US fortifications existed on both sides of the canal. On the Atlantic coast, the large coastal artillery batteries of Fort Sherman had a range of 30,000 yards (27430 m), preventing enemy ships from getting near enough to shell the locks. In the months following the attack on Pearl Harbor, air and sea patrols had been strengthened around both entrances, and barrage balloons and anti-submarine nets erected. In August 1942, the 88^th Coast (Anti-Aircraft) Artillery unit was added to help defend against air attacks.

As the war continued and Japan’s fortunes waned, security around the canal grew increasingly lax. In January 1944 Commander Fujimori personally interviewed a US prisoner of war who had done guard duty there: the American told Fujimori that defensive air patrols had virtually ceased, since it was considered increasingly unlikely the Axis powers would ever attack the locks. This further convinced Fujimori of his plan’s feasibility.

A Japanese engineer who had worked on the canal during its construction handed over hundreds of documents to the Naval General Staff, including blueprints of the canal structures and construction methods. A team of three shipping engineers studied the documents and concluded that the locks at Miraflores on the Pacific end of the canal were the most vulnerable to air attack, but that the Gatun locks on the Atlantic end offered the possibility of greater damage as it would be harder to halt any outflow of water. They estimated the canal would be unusable for at least six months following a successful attack on the locks.

To increase the size of the airborne attack force, Fujimori requested that two additional fleet submarines still under construction at Kobe, I-13 and I-14, be modified each to carry two Seiran aircraft, bringing the total number of aircraft available to 10. It was originally planned that two of the Seiran aircraft would carry torpedoes and the other eight would carry 800-kg (1,764-lb) bombs, and that these aircraft would deliver a combined torpedo and glide-bombing attack against the Gatun Locks. Eventually though, the torpedo-bombing task was abandoned as only one Seiran pilot had mastered the technique.

The Panama Canal strike plan called for four aircraft-carrying submarines (I-400, I-401, I-13 and I-14) to cross the Pacific to the Gulf of Panama, a journey expected to take two months. At a point 115 miles (185 km) off the coast of Ecuador, the submarines would launch their Seiran aircraft at 03.00 on a moonlit night. Not fitted with floats, the Seiran aircraft were then to fly at an altitude of 13,125 ft (4000 m) across the northern coast of Colombia to the vicinity of Colón. Now on the Caribbean side of the isthmus, they would turn to the west on a heading of 270°, then turn to the south-west and make their final approach to the canal locks at dawn. After completing their bombing runs, the Seiran aircraft were to return to a designated rendezvous point and ditch alongside the waiting submarines, which would recover the aircrews.

Around April 1945, Captain Ariizumi, the man appointed to carry out the attack, decided the Seiran pilots would make kamikaze ramming attacks against the gates rather than conventional bombing runs, a tactic becoming increasingly common as the war went against the Japanese. The Seiran squadron leader had already suggested as much to Ariizumi earlier that month, though for a time this was kept secret from the other pilots. At the end of May, however, one pilot happened to observe a Seiran having its bomb-release mechanism removed and replaced with a fixed mount. Appreciating the implications of the change, he angrily confronted the executive officer of the squadron, who explained that the decision to withhold this intention from the other men was made to 'avoid mental pressures on the aircrews'.

By 5 June 1945, all four aircraft-carrying submarines had arrived at Nanao Wan, where a full-scale wooden model of the Gatun Locks gate had been built by the Maizuru Naval Arsenal, placed on a raft and towed into the bay. The following night, formal training commenced with the Seiran flight crews practising rapid assembly, catapult launch and recovery of their aircraft. There was also rudimentary formation flying. From 15 June the Seiran pilots made practice daylight bombing runs against the wooden gate mock-up. By 20 June, all training ended and the operation was ready for implementation.

Before the attack could be launched, however, Okinawa had fallen, and word reached Japan that the Allies were preparing an assault on the Japanese home islands. The Japanese Naval General Staff concluded the Panama Canal attack, even if successful, would now have little impact on the war’s outcome, and more direct and immediate action was necessary to stem the US advance, and this led to the development of plans for the 'Arashi' attack on the US fleet, and specially its 15 aircraft carriers, anchored in the lagoon of Ulithi atoll.

During the last months of the war, meanwhile, Ishii developed a scheme for a long-range attack on the USA with biological weapons. This 'Cherry Blossoms at Night' was based on the deployment of aircraft to spread plague in the San Diego area in a nocturnal raid. The plan was finalised on 26 March 1945. Five of the new 'I-400' class long-range submarines were to be sent across the Pacific Ocean, each of the boats carrying three Aichi M6A Seiran single-engined aircraft loaded with plague-infected fleas. The submarines were to surface and launch the aircraft toward the target, either to drop the plague via balloon bombs, or to crash in US territory. Each of these scenarios was deemed adequate for the delivery of plague, which would then infect and kill many thousands of people.

The start of 'Cherry Blossoms at Night' was set for 22 September, but was not realised as a result of Japan’s surrender on 15 August 1945. General Yoshijiro Umezu, the chief of the army general staff, had at first vetoed the plan because of its logistical problems, but began to reconsider the concept at a time early in August 1945 after the Imperial Japanese navy had informed him that it had a few more capable long-range submarines to carry the aircraft without significant problems. Arata Mizoguchi, a Unit 731 commander in the Imperial Japanese navy, said that only three 'I-400' boats had been built by 15 August, but estimated that by 2 September two or three more would have been completed ahead of schedule had the war continued.