'Centerboard' was the US strategic atomic bombing of Nagasaki on the western side of Kyushu island in the Japanese home islands (9 August 1945).

'Centerboard' was the world’s second atomic bombing, following the 'Silverplate' attack on Hiroshima three days earlier. The detonation at 11.02 destroyed the northern part of the city and killed, out of a population of some 253,000 persons, 73,885 people as well as injuring another 74,910, of whom many died of radiation-related illnesses during the following decades.

Nagasaki is a major port and industrial city on the island of Kyushu, with large coal deposits in the nearby hills. These deposits were mined for 40% of Japan’s domestic coal. The city’s Mitsubishi shipyard was the largest in Japan, employing 36,391 workers and capable of building all types of ships. The Mitsubishi ordnance works were also located there, as well as two small aircraft factories. The port was protected by a heavy artillery fortress regiment.

The Nagasaki bomb, codenamed 'Fat Man', was dropped by a Boeing B-29 Superfortress four-engined heavy bomber named Bockscar (or Bock’s Car), and was a 22-kiloton implosion-type plutonium bomb more powerful than the 'Little Boy' 15-kiloton gun-type uranium bomb dropped on Hiroshima.

The mission was beset by a number of problems. The pump for the reserve fuel tank of the B-29 was malfunctioning but the mission was nonetheless ordered to proceed, for otherwise it would have been delayed for several days by an approaching storm front. One of the accompanying observation B-29 aircraft failed to make the rendezvous over the island of Yakoshima, causing Bock’s Car to expend more precious fuel circling and waiting for the missing aeroplane. Weather conditions were already deteriorating as Bock’s Car approached Japan, and three passes over the primary target, Kokura on the northern coast of Kyushu, failed because of heavy ground haze obscured the target.

Bock’s Car then turned toward Nagasaki with just enough fuel for a single pass before proceeding to Okinawa. Nagasaki was also obscured by poor weather, but the mission commander opted to disregard his orders not to bomb by radar, for otherwise the B-29 would have to land with the 'Fat Man' still on board or jettison the bomb into the sea.

The secondary target was the industrial area about 1.5 miles (2.4 km) to the north-west of the centre of Nagasaki. Bock’s Car's bombardier reported that the clouds cleared over the target just as the bomber was making his run, and the bomb detonated above a point some 500 ft (150 m) to the south of the Mitsubishi Steel and Arms Works in the Urakami district, some distance from the docks and city centre. The explosion also destroyed a Mitsubishi torpedo factory and the Urakami Roman Catholic cathedral.

Despite the use of a more powerful bomb, the devastation visited upon Nagasaki was less severe than that experienced by Hiroshima, largely as a result of an aiming point away from the city centre and Nagasaki’s very hilly topography, which comprises a series of narrow valleys enclosed by eastern and western mountains that served to channel the blast of the detonating atomic bomb. The parachute-retarded bomb detonated at an altitude of 1,640 ft (500 m) directly above the suburb of Urakami, and the result was the devastation of some 2.6 sq miles (6.7 km²), including the destruction or severe damage of 11,580 of the city’s 51,000 buildings.

After its attack, Bockscar was so low on fuel that the flight to Okinawa was made in a series of carefully calculated 'steps', in which altitude was traded for speed to stretch the fuel supply. One engine stopped from fuel starvation as the aeroplane made its landing approach, a second stopped on the runway, and a third stopped just as the aeroplane came to a halt. Bockscar had 7 US gal (26.5 litres) of fuel left excluding the 600 US gal (2271 litres) trapped in the tank with the defective pump.

The US political and military leadership felt that the early use of a second nuclear weapon had been necessary to convince the Japanese that more such weapons were available in the US stockpile. In fact, 'Manhattan Project' leaders estimated that seven more bombs would have been available by 31 October 1945. Tokyo was to have been the next target, but intelligence reports of a massive Japanese build-up on Kyushu led General George C. Marshall, the US Army’s chief-of-staff, to request that the remaining bombs be reserved for tactical use in case the 'Olympic' invasion of Japan had to be ordered.