[2] The 129th Infantry arrived at the port of New York on 23 May 1919 on the USS Leviathan as an element of the 33rd Division, and was demobilized 6 June 1919 at Camp Grant, Illinois.

It was reconstituted in the National Guard in 1921, assigned to the 33rd Division, and allotted to the state of Illinois.

The regiment, or elements thereof, was called up to perform the following state duties: tornado relief work at Carbondale, Illinois, 18 March–13 April 1925; martial law in connection with civil disorders in Mundelein, Illinois, 23–25 June 1926; three companies for riot control at the Joliet Prison Riot, 18–23 March 1931; 3rd Battalion acted as honor guard for President Herbert Hoover at the rededication of the Lincoln Tomb at Springfield, Illinois, 17 June 1931; entire regiment for strike duty at a coal miners’ strike at Springfield and Taylorville, Illinois, in March–April 1933; and three companies for strike duty at a labor strike in Freeport, Illinois, in June 1935.

For at least one year, in 1936, the regiment also trained some 15 company-grade infantry officers of the 86th Division at Camp Grant.

The regimental headquarters was organized and federally recognized on 18 December 1946 at Dixon, Illinois.