Lineage

• Constituted 3 May 1861 in the Regular Army as the 2nd Battalion, 12th Infantry
• Organized 20 May 1862 at Fort Hamilton, New York
• Reorganized and redesignated 7 December 1866 as the 21st Infantry
• Consolidated 9–31 August 1869 with the 32nd Infantry (see ANNEX) and consolidated unit designated as the 21st Infantry
• Assigned 29 July 1918 to the 16th Division
• Relieved 8 March 1919 from assignment to the 16th Division
• Assigned 22 October 1921 to the Hawaiian Division (later redesignated as the 24th Infantry Division)
• Relieved 5 June 1958 from assignment to the 24th Infantry Division and reorganized as a parent regiment under the Combat Arms Regimental System
• Withdrawn 16 January 1986 from the Combat Arms Regimental System and reorganized under the United States Army Regimental System

Annex

• Constituted 3 May 1861 in the Regular Army as the 3d Battalion, 14th Infantry
• Organized 27 July 1865 at Fort Trumbull, Connecticut
• Reorganized and redesignated 21 September 1866 as the 32d Infantry
• Consolidated 9–31 August 1869 with the 21st Infantry and consolidated unit designated as the 21st Infantry

Campaign participation credit

• Civil War: Peninsula; Manassas; Antietam; Fredericksburg; Chancellorsville; Gettysburg; Wilderness; Spotsylvania; Cold Harbor; Petersburg; Virginia 1862; Virginia 1863
• Indian Wars: Modocs; Nez Perces; Bannocks; Arizona 1866; Arizona 1867; Arizona 1868; Arizona 1869; Arizona 1870
• War with Spain: Santiago
• Philippine–American War: Zapote River; Luzon 1899; Luzon 1901; Luzon 1902
• World War II: Central Pacific; New Guinea (with arrowhead); Leyte; Luzon; Southern Philippines (with arrowhead)
• Korean War: UN Defensive; UN Offensive; CCF Intervention; First UN Counteroffensive; CCF Spring Offensive; UN Summer-Fall Offensive; Second Korean Winter; Korea, Summer 1953
• Vietnam: Counteroffensive, Phase II; 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
• Armed Forces Expeditions: Panama
• Operation Uphold Democracy
  Operation Uphold Democracy
  Operation Uphold Democracy was an intervention designed to remove the military regime installed by the 1991 Haitian coup d'état that overthrew the elected President Jean-Bertrand Aristide...
  
  , Port-Au-Prince, Haiti, 1995
• Operation Iraqi Freedom and Enduring Freedom (Afghanistan), Global War on Terrorism

Decorations

• Valorous Unit Award for OIF 3 August-October 2004-2005
• Presidential Unit Citation (Army) for DEFENSE OF KOREA
• Presidential Unit Citation (Army) for SANGHONGJONG-NI
• Valorous Unit Award for TAM KY - TIEN PHUOC
• Navy Unit Commendation for CAM LO
• Philippine Presidential Unit Citation for 17 OCTOBER 1944 TO 4 JULY 1945
• Republic of Korea Presidential Unit Citation for PYONGTAEK
• Republic of Korea Presidential Unit Citation for KOREA

21st Infantry Korean War casualty