The Mitsubishi A6M Zero-Sen

The "Zeke" or "Zero" to the Allies

The Zero was the emerging standard fighter for the Japanese at the beginning of their Pacific campaign; it was fairly new (having first flown in 1939) but not so new the U.S. didn't know about it - it had seen action in China and elsewhere long before it was used against the Americans. Even so American armed forces severely underestimated the A6M and that mistake caused considerable problems - it's primary opponents in the early war, the F4F Wildcat, the P40, and the lamentable P39, were heavily outclassed by the Zero.

The wide disparity in plane ability forced a quick adoption of new air to air tactics by the American pilots; the famous "Thatch weave" for instance, which was effective but required two to one engagement odds, close wingman contact, and was fairly risky even so. More commonly Americans took to a simple philosophy echoed by pilot biographies throughout the era; engage with altitude, engage with numbers, and when compromised immediately dive to safety (a singular weakness of the Zero was high speed diving).

This confused the Japanese, who by and large considered air combat almost an art form which matched the wits and ability of one pilot against another. 

This was mirrored in the very design of the Zero; it was at the time the ultimate "dogfight" style plane, exhibiting superb turning capability, good range, and excellent power - at the price of durability, low offensive armament, and very limited payload or bombing options. If American pilots had attempted to engage the Japanese on a level playing field, so to speak, they would have been slaughtered, both by the superior design and better training at the onset of the war the Japanese enjoyed.

Unfortunately for the Japanese the features they designed their premier fighter for were quickly obsolete in the air war. Having demonstrated the validity of their new tactics in combat, the U.S. armed forces began making planes to better utilize them, and training their pilots extremely well in how to do it. Against a supreme turning plane the U.S. answered with several planes that had common core design features: they were fast, they were heavily armed, they were heavily armoured, and they performed well at high altitudes. 

Against the likes of well flown P38, F6F or F4U, the Zero was hopelessly outclassed, but by the time that was evident to all concerned it was too late to displace it as the number one fighter for Japan. When asked after the war which plane he fly in WW2 combat if he had a choice of any, Sabaro Sakai, one of the leading Japanese aces of WW2, said he would pick a P51. The pilots of the Zero understood what they needed was a faster, tougher plane, but the design that produced such outstanding early war results was no longer able to give them that four years later.