In WW2, the Japanese Zero did not have armor on the theory that the lighter weight would make the Zero more maneuverable and that was the best defense a pilot could have.

The Zero's first adversary was the P40 Warhawk. The P40 was inferior in every way but one - it had forward armor. The pilots figured out that the way to beat the Zero was a head on attack, with both pilots blasting away. With no armor the Zero would get shredded. The Zero pilots had a pretty hard time of it during the war.