At the end of World War I, the return of peace brought a sharp decline in orders for military equipment among the victorious powers. Japan was one of them, having been an ally of Great Britain. However, rather than turning toward peace, the Japanese government launched a new buildup, in preparation for future wars. In 1920 alone, the Japanese army and navy placed orders for nearly 1,400 warplanes. During the next two decades, and during World War II, the firm of Mitsubishi stood in the forefront of Japanese military aviation.

Mitsubishi was a "zaibatsu," which translates as "wealthy clique." It was a family-owned industrial combine that owned banks, which provided it with funds. Mitsubishi was the second largest zaibatsu, standing alongside the Mitsui, Sumitomo, and Yasuda zaibatsu. Founded in 1873 by the entrepreneur Iwasaki Yataro, it received strong government support from the outset. It started with trading and merchant shipping, then expanding into mining, shipbuilding, real estate, iron and steel, insurance, oil refining, and chemicals.

The group entered aviation during World War I, building French aircraft engines under license and soon producing trainer aircraft that were also French. A design group headed by Britain's Herbert Smith, who had been a chief engineer at Sopwith Aviation Company, crafted new warplanes that became standard equipment with the Japanese navy. Mitsubishi also learned lessons from Germany, first by working with the aircraft designer Alexander Baumann and then through a collaboration with the German planebuilding firm of Junkers.

Preparations for war went forward rapidly during the 1920s, as the Navy built its first aircraft carriers. These included the 800-foot (244-meter) Akagi and Kaga, which were among the world's largest. In 1930, the naval leader Isoroku Yamamoto torpedoed international plans to limit construction of warships, thereby giving Japan free rein to continue its buildup. By then, the United States had powerful carriers of its own, and Yamamoto saw them as a threat. He demanded construction of long-range bombers that could strike those Yankee warships. Mitsubishi responded with a twin-engine aircraft that later became known as the Nell.

Japan went to war in 1931, initially against China. In August 1937, with the war escalating, a force of 38 Nells flew from Japan and struck Chinese targets 1,250 miles (2,012 kilometers) away. This was astonishing, for its range was more than double that of any other bomber then flying. Mitsubishi achieved this performance by making the plane light in weight by removing guns and armour. Indeed, pilots were told that a desire for armour was a sign of cowardice.

Two years later, the company introduced what became Japan's standard twin-engine bomber: the Betty. Mitsubishi provided the engines as well. The Betty could fly well over 3,000 miles (4,828 kilometres) without refuelling, which again it achieved by eliminating armour and fire-resistant fuel tanks to save weight. The firm went on to build nearly 2,500 of them.

During 1939, Mitsubishi also launched Japan's most famous and deadliest wartime fighter: the Zero. Again, light weight was its strong suit. An early opponent, the Curtiss P-40, weighed 1,000 pounds (454 kilograms) more when empty than the Zero did when loaded. This gave the Zero great manoeuvrability. It carried two machine guns and two cannons that fired 20-millimeter shells. More than 10,000 were built, while the Allies had no aircraft that could match it until 1943.

These warplanes played vital roles in a sweeping Japanese offensive that got under way in December 1941. On December 7, a fleet of six aircraft carriers struck Pearl Harbor, a major U.S. naval base. The attack delivered a heavy blow against the Pacific Fleet and led the next day to a declaration of war against Japan.

Hours after the attack on Pearl Harbour, a land-based force of Mitsubishi Nells and Bettys, escorted by Zeroes, struck U.S. airfields in the Philippines. The Zeroes did particular damage, firing at parked aircraft with their guns. The Japanese destroyed more than 100 U.S. warplanes. The American forces never recovered, as Japan invaded the main island of Luzon. The defeat that followed was one of the most bitter in American history.

Then came the turn of the British. They held a vital naval base at Singapore and sought to reinforce it by sending the battleship Prince of Wales and the battle cruiser Repulse. A large force of Nells and Bettys caught them in the open and bombed them. Japanese torpedo planes finished them off, with 14 torpedoes striking the Repulse and seven hitting the Wales.

General Sir Alan Brooke, chief of Britain's General Staff, wrote that loss of those warships meant that "from Africa eastward to America through the Indian Ocean and the Pacific, we have lost control of the sea." Singapore was Britain's key position in the Far East, but the sinking of those ships left its defenders without hope. They outnumbered the attacking Japanese, 70,000 to 35,000, but two months later this British force surrendered.

In the wake of these triumphs, Japan's empire included Southeast Asia and much of China, while extending southward toward Australia and eastward far into the Pacific. As the war continued, Mitsubishi became that nation's leading builder of aircraft engines. It ranked number two in number of aircraft produced, with the firm of Nakajima in first place, but Mitsubishi held particular strength in production of bombers.

Despite the breadth of its conquests, Japan's position was vulnerable, with its vast empire being open to counterattack. Yamamoto himself had foreseen this. Prior to the war he stated, "If I am told to fight regardless of the consequences, I shall run wild for the first six months or a year, but I have utterly no confidence for the second and third years of the fighting." He had seen what he described as "the automobile factories in Detroit and the oil fields in Texas," and he was well aware that Japan lacked such industrial strength. He knew that in a long war, the United States would mobilize this strength to build powerful weapons in numbers that Japan could not match.

Following the surrender of Japan, in 1945, American authorities broke up the zaibatsu, including Mitsubishi. These industrial combines later reorganized as corporations set up in the American style, without the family control and ownership of the pre-war decades. Portions of Mitsubishi remained active in building power plants and heavy machinery, in chemicals, and in banking and overseas trade. The company also expanded into new areas, winning a strong position in electronics and in automobiles and light trucks.