Task Force Smith, the first American unit to see action in the Korean War, was derived from the regiment's 1st Battalion.

The companies were formed into a battalion with elements of the 8th Infantry Regiment and were the lead skirmishers of the II Corps' 2nd Division.

[2] The regiment was brought up to full strength at Fort Hamilton and moved back to Virginia for reconstruction duties by December 1865.

[4] The regiment continued to pursue the Indians through the Bitterroot Valley to the Bears Paw Mountains, where Chief Joseph finally surrendered.

After marching 35 miles in a day, the regiment overtook the tribe on 13 August, scattering it so that it no longer posed a threat and had to return to the reservation.

[2] In December 1916, second battalion of the regiment participated in the Panama–California Exposition, defending against a simulated attack from two Navy cruisers, Frederick and San Diego, and several aircraft.

[2] The 21st Infantry, less the 2nd and 3rd Battalions, was stationed at Fort George Wright, Washington, as of June 1919 as a separate regiment.

The primary wartime mission of the 21st Infantry was to conduct a mobile defense of the beaches and inland sectors of the northern half of the island of Oahu.

The regiment captured the northern coast of Panaon Island and the southern shore of Leyte without meeting resistance, an hour before the main landings farther to the north.

For its fire support of the division, Cannon Company was awarded a Presidential Unit Citation for actions between 31 January and 5 February.

A battalion of the regiment crossed the Talomo River near Mintal on 8 May but had to withdraw two days later due to Japanese resistance.

[2] On 12 July, a battalion combat team of the 21st landed on the northwest shore of Sarangani Bay, where Japanese troops still held out.

Along with other American & Filipino military units and recognized guerrillas, the battalion combat team cleared the Japanese from the area.

On 30 June, President Harry Truman decided to defend South Korea with American ground forces.

[2] Task Force Smith airlifted into Pusan and then travelled by rail and truck to a position north of Osan, 45 kilometers south of Seoul.

At 08:16 on 5 July, the task force opened fire on a column of 33 KPA T-34/85 tanks, supported by two regiments of infantry.

Due to the lack of anti-tank weapons, the task force could only destroy four T-34/85s as the tank column continued south.

[2] During the Battle of Osan, Task Force Smith suffered casualties of 60 killed, 21 wounded and 80 captured, out of a total strength of 540.

C Company Sergeant First Class Ray E. Duke was posthumously awarded the Medal of Honor for actions near Mugok on 26 April.

During fighting between 14 and 15 November, G Company captured two heavily defended hills against PVA resistance, earning it a Presidential Unit Citation.

K Company Private First Class Mack A. Jordan was posthumously awarded the Medal of Honor for his actions near Kumsong on 15 November.

The Korean Armistice Agreement was signed on 27 July, and the regiment oversaw POW exchanges at the camp.

After the end of Operation Big Switch, the regiment was moved into reserve positions behind the Demilitarized Zone.

[15] 3rd Battalion C Company medic James McCloughan received the Medal of Honor in 2017 for his actions near Tam Kỳ between 13 and 15 May.

From 5 to 11 August 1972, Delta Company, 3/21st Infantry undertook the last patrol by U.S. troops in the war with two soldiers wounded by booby traps.

The brigade arrived in Iraq for a fifteen-month tour of duty in November 2007and was based at Camp Taji northwest of Baghdad.

Serving with the Multi-National Division-Baghdad, the brigade was responsible for the rural areas northwest and west of Baghdad with the 1st Battalion operating near Abu Ghuraib.

The 1st Battalion, working closely with their Iraqi counterparts, was especially successful in eliminating terrorist cells and uncovering and destroying multiple weapons caches.

In February 2016, soldiers of the regiment's first battalion participated in Exercise Lightning Forge with other units of the 25th Division's 2nd Stryker Brigade Combat Team.

[19] Between 2019 and 2020, 3rd Battalion deployed with 1st Stryker Brigade Combat Team, 25th Infantry Division out of Fort Wainwright, Alaska in support of Operation Inherent Resolve.