The Mitsubishi A6M Zero-sen is the most famous Japanese fighter of the Second World War (1939 to 45). The single-seater aircraft was a leap forward for the Japanese air force and navy; indeed, this was the fastest and most manoeuvrable of all fighter planes in the early years of the war. Deployed very often from aircraft carriers, the Zero was used as both a fighter and dive-bomber in such key battles as the attack on Pearl Harbor and the Battle of Midway. Although outclassed in the final two years of the war by Allied fighters, production of the Zero continued, and it became the most produced of any aircraft by any air force in WWII.

The Japanese Imperial Navy wanted a more modern fighter to replace the Mitsubishi A5M, which had been in service from 1936 but could not match rival fighters of other air forces. Accordingly, Designer Jiro Horikoshi (1903 to 1982) came up with a much sleeker aircraft than the A5M, which had rather old-fashioned lines not very much different from a biplane, even having an open cockpit. The A6M was given the designation Type O Carrier-Based Fighter. The Allies during the Second World War officially gave the A6M aircraft the code name "Zeke," but pilots simply called them "Zeros."

The Zero had a covered cockpit, distinctive cantilever low wings, and was almost all-metal in construction. The tail wheel could be retracted after takeoff. The first planes, developed through 1939, satisfied the navy, but the Mitsubishi engine needed more power if the fighter was to compete with the best fighters out there. Consequently, successful tests were made using a Nakajima NK 1C Sakae 12 radial engine. The Nakajima engine was heavier but gave around 145 hp (108 kW) more power than its Mitsubishi predecessor. The extra weight of the engine was somewhat compensated for by the overall lightness of the Zero fighter. The Zero was faster than Britain's best, the Supermarine Spitfire, and Germany's best, the Messerschmitt Bf 109.

The Japanese navy ordered 15 of the Nakajima-powered aircraft in July 1940 for operations in Japanese-occupied China. Success in action in China led to more orders. The Zero was fast and, thanks to its lightweight construction, possessed a high degree of manoeuvrability, attributes which meant Chinese air defences struggled to cope with this new menace. As the historian D. Marston notes, the Zero was "the fastest, most maneuvrable land- or sea-based fighter operated by any air-force during the first two years of World War II" (95).

The Japanese navy began to accumulate large numbers of Zero fighters for use on their aircraft carriers. These large orders necessitated sharing production between Mitsubishi and Nakajima factories. The A6M2 Model 21 version was given folding wingtips (moved manually) to save space on the carriers when the planes were bunched up below decks. Even better, the A6M3 redesigned the wingtips to be clipped shorter to avoid the need to have complicated folding parts. However, it was found that in actual operation this design tweak reduced the range of the aircraft, and so the folding wingtips were reinstated. The A6M5 returned to non-folding wings but was given more rounded wingtips.

No doubt to the relief of pilots, the A6M5b was given an armoured glass canopy. The A6M6e had self-sealing fuel tanks added. Further developments in engines, such as adding a turbo-charge and water-methanol-injection system, gave ever greater speeds as the Zero evolved through the early years of the war, although the extra weight of the more powerful engines compromised the aircraft's manoeuvrability. There was also a two-seater version, the A6M2-K, which was used as a training aircraft. In production right through the war, 10,449 A6M Zeros were produced in all, more than any other aircraft type of World War II.

No other armed services used the Zero, except in China during WWII, where captured fighters were sometimes repainted and flown to face the Japanese armed forces. Horikoshi, the Zero's designer, tells the full history of the aircraft's development in his 1981 book, Eagles of Mitsubishi: The Story of the Zero Fighter.

The standard Zero was 29 feet 9 inches (9.07 m) long and had a wingspan of a little over 36 ft (11 m). The fastest Zero fighters were those fitted with Nakajima Sakae 31 14-cylinder radial piston engines, which could provide around 1,130 hp (843 kW). The top speed was around 346 mph (557 km/h), although closer to 300 mph was more typical. The service ceiling of the A6M was around 35,000 feet (107,000 m). The maximum range was a little over 1,100 miles (1,800 km). As with other fighter aircraft, the Zero could be adapted to carry fuel drop tanks to extend its range to around 1,930 miles (3,106 km).

The standard armament was two or three machine guns, each with a calibre of 0.52 inches (13.2 mm). These guns at first had 60-round magazines but were later improved to hold 100 rounds. One gun could be set in the upper fuselage and one in each wing. Each wing also had a 0.78-inch (20-mm) cannon. When operating as a dive-bomber, the Zero could be fitted with either eight 22-lb (10-kg) rockets or two 132-lb (60-kg) rockets under the wings. Some later Zeros were adapted to carry a single 551-lb (250-kg) bomb.

By late 1941, the Japanese Navy had around 400 Zero fighters. 125 Zero fighters, flying out from six different aircraft carriers, took part in the attack on Pearl Harbor, Hawaii, the base of the US Pacific Fleet, in December 1941. The Zero fighters were tasked with escorting the more vulnerable bombers and dive-bombers to their targets, and so Zeros were usually the first planes to take off from a carrier. Later in an operation, when their escort duties were over, Zeros could also be used to attack enemy bombers, ships, and fighters. Each Japanese carrier had a squadron of around 18 fighter aircraft and two squadrons of bombers, each with 27 aircraft. Zeros were not restricted to naval use but also operated from land bases across Japanese-occupied territory from Manchuria in northern China to Truk Atoll in the middle of the Pacific Ocean.

Zeros again saw action in the Battle of the Coral Sea in May 1942. This was history's first carrier battle, and the range, speed, and manoeuvrability of the Zero fighters surprised the US pilots. The only Allied aircraft capable of matching the Zero was the USAAF's Grumman F4F Wildcat, another carrier-based fighter (although later, heavier versions of this fighter were less suited to direct confrontations with the nippier Zero). Japanese fighters typically flew in a group of three (a shotai), with each aircraft staggered in a line or echelon formation.

Zeros again flew in the Battle of Midway in June 1942, but by then the US fighter pilots were adapting. As the National Museum of the US Air Force notes, Allied pilots began to avoid dogfights with Zeros and instead adopted different tactics: "The key to fighting the Zero was to stay out of dogfights, and instead use superior armament and hit-and-run diving attacks against the relatively fragile A6M." The lightweight build of the Zero meant there was next to no protection for either the pilot or fuel tanks from even small-calibre enemy fire. Another weakness was the Zero's poor-quality onboard radio, which meant that it was not usually possible to communicate changes of orders once the aircraft was airborne.

From 1943, the capabilities of US fighters improved and began to reverse the Zero's dominance in the air. Faster engines and increased firepower were seen in such aircraft as the North American P-51 Mustang. It was also true that the US was putting many more aircraft into the air than Japan was capable of doing, while, conversely, Japan began to see an attritional decline in its number of experienced pilots. The desperate need for more pilots forced the Japanese to rush their pilot training programmes so that many young men in the combat zone were still struggling to properly manage their aircraft.

From October 1944, Zeros, which were almost obsolete compared to the best US fighters, were frequently used for kamikaze missions, first seen in the Battle of the Philippines. In these missions, volunteer pilots deliberately flew their aircraft, which were typically modified to carry a heavier-than-usual bomb load such as a single 551 lb (250 kg) bomb, directly into targets like warships. Zeros were used more than any other type of plane on these missions, around 650 of them. Zeros were also deployed to escort kamikaze aircraft to their final destination. Considered a great honour, over 5,000 Japanese pilots died in this way during the war.

Ultimately, the kamikaze campaign was not effective. The Allies continued to push Japanese forces right back to the home islands. Overall, 47 Allied ships were sunk and 300 damaged by kamikaze pilots, but this number was relatively small in the grand scheme of the Pacific War. The strategy was most successful early on, before the Allies realised pilots were deliberately crashing their aircraft even though they were undamaged. At the Battle of the Philippines, 27% of the kamikaze attacks caused damage to Allied ships. At the Battle of Okinawa, although 36 ships and landing craft were sunk and another 369 damaged, the success rate had fallen to just 15%. Thereafter, only a small percentage of kamikaze aircraft survived intense Allied fighter defences and anti-aircraft fire using special explosive rounds. Even those that managed to penetrate the defences typically damaged only minor shipping that could be easily replaced. It was an ignominious end for the Zero fighter that had so dominated the early years of the war.