During the interwar period, it was reconstituted in the Organized Reserve with the 88th Division and later served in the Mediterranean Theater during World War II.

The regiment typically conducted inactive training period meetings at the National Guard armory in St. Paul and at the American Legion Club in Austin.

[8][9] The regiment was ordered into active military service 15 July 1942 and reorganized at Camp Gruber, Oklahoma using a cadre provided by the 9th Infantry Division.

In July 1943, the regiment was organized with 3,256 officers and enlisted men:[12] The 351st Infantry was relieved from assignment to the 88th Infantry Division on 1 May 1947 and served as temporary military Government of the Free Territory of Trieste, securing the new independent State[13] between Italy and Yugoslavia on behalf of the United Nations Security Council.

[14] Designated TRUST (Trieste United States Troops), the command served as the front line in the Cold War from 1947 to 1954, including confrontations with Yugoslavian forces.

In October 1954 the mission ended upon the signing of the Memorandum of Understanding of London [15] establishing a temporary civil administration in the Anglo-American Zone of the Free Territory of Trieste, entrusted to the responsibility of the Italian Government.

[citation needed] The 351st Infantry was redesignated as the 351st Regiment, and reorganized to consist of the 1st, 2nd, and 3rd Battalions, elements of the 84th Division (Training) on 31 January 1968.