Constituted 3 May 1861 in the Regular Army as the 2d Battalion, 11th Infantry
Organized 6 June 1862 at Fort Independence, Massachusetts
Reorganized and redesignated 6 December 1866 as the 20th Infantry
Assigned 9 July 1918 to the 10th Division
Relieved 14 February 1919 from assignment to the 10th Division
Assigned 18 September 1920 to the 2d Division
Relieved 16 October 1939 from assignment to the 2d Division and assigned to the 6th Division (later redesignated as the 6th Infantry Division)
Inactivated 10 January 1949 in Korea
Activated 4 October 1950 at Fort Ord, California
Relieved 3 April 1956 from assignment to the 6th Infantry Division
Reorganized 15 November 1957 as a parent regiment under the Combat Arms Regimental System
Withdrawn 16 August 1986 from the Combat Arms Regimental System and reorganized under the United States Army Regimental System

Civil War (as 2nd Bn, 11th Infantry Regiment)
Peninsula
Manassas
Antietam
Fredericksburg
Chancellorsville
Gettysburg
Wilderness
Spotsylvania
Cold Harbor
Petersburg
Virginia 1862
Virginia 1863

Indian Wars (as 20th US Infantry)
Little Big Horn
Pine Ridge

War with Spain (as 20th US Infantry, V Corp)
Santiago

Philippine Insurrection
Manila
Luzon 1901
World War II
New Guinea
Luzon (with arrowhead)

Vietnam (as 1st Bn, 20th Infantry, 11th Light Infantry Brigade, 23rd Americal)
Counteroffensive, Phase III
Tet Counteroffensive
Counteroffensive, Phase IV
Counteroffensive, Phase V
Counteroffensive, Phase VI
Tet 69/Counteroffensive
Summer-Fall 1969
Winter-Spring 1970
Sanctuary Counteroffensive
Counteroffensive, Phase VII
Consolidation I
Consolidation II
Cease-Fire

Presidential Unit Citation (Army), Streamer embroidered MAFFIN BAY

Presidential Unit Citation (Army), Streamer embroidered CABARUAN HILLS

Presidential Unit Citation (Army), Streamer embroidered MUNOZ

Meritorious Unit Commendation (Army), Streamer embroidered VIETNAM 1972

Philippine Presidential Unit Citation, Streamer embroidered 17 OCTOBER 1944 TO 4 JULY 1945