Correspondence, memoranda, speeches, writings, reports, military orders, patents, newspaper clippings, printed matter, scrapbooks, drawings, photographs, and other papers pertaining primarily to Kowalski's career in the U.S. Army (1925-1958) and in the U.S. House of Representatives (1959-1963).

Military files document his directorship of the Disarmament School, U.S. Army Forces in the European Theater, London, England (1944-1945) and the school's training of Allied and American officers for the demobilization and disarmament of Germany; his various assignments during the U.S. occupation of Japan (1948-1952), particularly as chief of staff of the American advisory group overseeing the establishment of the Japanese National Police Reserve (Keisatsu Yobitai) at the outbreak of the Korean War; and his directorship of the Army Command Management School, Fort Belvoir, Va. (1954-1958).

Congressional files document his work relating to several labor disputes in Connecticut, his interest in military reform as a member of the House Committee on Armed Services and its Special Subcommittee on the Utilization of Military Manpower, and his opposition to the Vietnam War. Postcongressional files relate to his membership on the Subversive Activities Control Board (1963-1966).

Includes manuscripts describing the economic, political, and social conditions in Poland (1945); a report (1944) based on interviews of Polish-born soldiers who served in the German army; and fragments of Kowalski's manuscript examining U.S. occupation and rearmament of Japan titled, Grace of Heaven, and published as Niho Saisumbi (Tokyo, 1969); and an unpublished manuscript titled, Worms in Charter Oak, concerning Kowalski's political career and his relations with the Democratic state chairman of Connecticut, John M. Bailey.

Correspondents include Bunzō Akama, William Benton, Chester Bowles, Wilber Marion Brucker, Arleigh A. Burke, Chester R. Davis, Keizo Hayashi, William Bradford Huie, Robert F. Kennedy, Hiroo Konda, Keikichi Masuhara, Wilbur D. Mills, Hirokichi Nishioka, Adam Clayton Powell, Sam Rayburn, Abraham Ribicoff, Yoshizo Takeda, and Maxwell D. Taylor.