The unit's first operation was an unopposed landing at Nadzab, in the Markham Valley, New Guinea, on 5 September 1943. Although the landings were unopposed, the troops were later attacked by enemy bombers from the air. The 503rd's deployment helped force the Japanese evacuation of a major military outpost at Lae. During their overland withdrawal, the third battalion of the 503rd had a major skirmish with the Japanese rear guard.