The 442nd Regimental Combat Team was actually composed of two distinct units: the 442nd Regimental Combat Team and the 100th Infantry Battalion. These two units were formed independently at different times and do not share a common lineage. The 100th Battalion would eventually become the 442nd's 1st battalion in June 1944.

The 100th Infantry Battalion (Separate) was the first U.S. Army unit of Japanese Americans activated in World War II. The 100th Battalion began its existence as the Hawaiian Provisional Infantry Battalion that was activated on June 5, 1942 in Hawaii. The soldiers of the Hawaiian Provisional Battalion came from various units of the Hawaiian National Guard. The Hawaiian Provisional Infantry Battalion was transferred to the mainland and arrived in San Francisco on June 12, 1942. The unit was then designated the 100th Infantry Battalion (Separate). The 100th Battalion trained at Camp McCoy, Wisconsin, and then at Camp Shelby, Mississippi. The 100th Battalion left Camp Shelby on August 11, 1943 bound for Oran, North Africa. On September 22, 1943 the 100th Battalion landed at Salerno, Italy while attached to the 133rd Infantry Regiment, 34th Division.

The 442nd Regimental Combat Team was activated on February 1, 1943 at Camp Shelby Mississippi. The 442nd was comprised of the 442nd Infantry Regiment; the 522nd Field Artillery Battalion; the 232nd Combat Engineer Company. The 442nd Regimental Combat Team was composed of Japanese American volunteers from the internment camps, Hawaii, states outside of the west coast exclusion zone, and Japanese American soldiers who were already serving in the U.S. Army when the war broke out. These Japanese American soldiers already in the Army would become the cadre for the new 442nd RCT.

The 442nd RCT trained at Camp Shelby and left on April 22, 1944 bound for Italy. During its training at Camp Shelby the 442nd supplied the 100th Battalion with replacement personnel which depleted the ranks of the regiment. When the 442nd left Camp Shelby for Italy it was comprised of the 2nd and 3rd Battalions as it left the reduced staffed 1st Battalion at Camp Shelby to train replacements for the regiment.

The 442nd arrived in Italy in June 1944 and was attached to the 34th Division on June 10, 1944. On this same day the 100th Battalion was attached to the 442nd to take the place of the regiment's missing 1st Battalion. On August 2, 1944, the 100th Battalion was formally redesignated and reorganized as the 100th Battalion, 442nd Infantry. For unit "morale and historical significances", the 100th Battalion was allowed to retain its original designation instead of being redesignated as the 1st Battalion.

The designation of the unit remained the 442nd Regimental Combat Team which was comprised of the 100th Battalion, 2nd Battalion, 3rd Battalion, 522nd Field Artillery Battalion, and the 232nd Combat Engineer Company. Thus, the 442nd Regimental Combat Team was formed and its first day in combat was on June 26, 1944. By this date the 100th Battalion had already been in the Mediterranean Theater for over 9 months and had been involved in many significant battles including the Battle of Casino. After the two units merged, the 442nd RCT and the 100th Battalion were a single infantry regiment with a single history.