The Kawasaki Ki-61 Hien ("Swallow") fighter represented a major departure for Japanese aircraft design in World War II. While other Japanese fighters were designed with air-cooled radials and were optimized for maneuverability, the Ki-61 used a liquid-cooled in-line engine and was designed for speed and power. In fact, the Ki-61 was so different from other Japanese fighters that when the type was first encountered in combat over New Guinea in June 1943, the Allies thought it wasn't a Japanese design at all. At first they believed it was a copy of the German Messerschmitt Me-109, then suspected it was a copy of the Italian Macchi C.202 Foglore or similar Italian fighter. For this reason they gave it the code-name "Antonio", or "Tony", though by the summer of 1943 the Allies had realized the Ki-61 was in fact a Japanese design. The Hien proved initially successful in combat against American fighters. As the war in the Pacific ground on, however, the Ki-61 found itself increasingly outclassed, but it soldiered on until the end of hostilities. | 
