The squadron flew its first combat mission on 5 June 1944 against the Makasan railroad yards at Bangkok, Thailand. Ten days later the 678th participated in the first American air attack on the Japanese home islands since the 1942 Doolittle raid, staging through Chinese bases on a nighttime raid against the iron and steel works at Yawata, Japan. It returned to Yawata on 20 August on a daytime raid for which the unit was awarded the Distinguished Unit Citation. Operating from bases in India and at times staging through fields in China, the group struck transportation centers, naval installations, aircraft plants and other targets in Burma, China, Thailand, Japan and Formosa.