Following the US Civil War, the Army was reorganized by Congress in July 1866, and the 11th was divided into three regiments, each battalion receiving two additional companies and being organized along traditional lines.

From 1986 to 1995 the 5th Battalion, 20th Infantry (Mechanized) were responsible for conducting patrol missions along the Korean DMZ during the Cold War.

Once the Cold War ended the battalion was pulled off of their permanent position along the DMZ, and conducted stability and support operations (SASO) throughout South Korea.

[4] A helicopter crew from the division's 123rd Aviation Battalion, led by Hugh Thompson, Jr., attempted to intervene in the massacre and were later awarded the Soldier's Medal.

[6] For his part, Calley was charged, convicted and sentenced to life imprisonment and hard labor on 31 March 1971 for the murder of 22 South Vietnamese civilians.

Calley, the only person convicted for the slaughter of hundreds of innocent South Vietnamese civilians at My Lai, eventually served only three and half years of house arrest and was released in September 1974.

The transformation began in 1999 with the conversion of the 3rd BDE, 2nd Infantry Division at Fort Lewis, WA to the Army's first Stryker brigade.

Soldiers of 5th Battalion, 20th Infantry took part in Operation Sykes Hammer and patrolled the streets of Tal Afar, August 2004.

They worked with the Iraqi National Guard in executing cordon and search operations in the neighborhoods of Tal Afar, which was successful in detained personnel, weapons and propaganda materials.

The 3rd Brigade as a whole were called on to support major operations when violence heated up in Al Kut, Tal Afar and Najaf.

After coming home and going through their second reset the infantrymen of 5th Battalion deployed with the 3rd BDE, 2nd Infantry Division from June 2006 to October 2007.

While in Mosul Bronco Troop, 1-14 CAV became task organized to 5-20 Infantry in Kuwait in exchange for Charlie Company.

In late November 2006, TF 5-20 Infantry conducted a ground assault convoy from Mosul to Taji, Iraq.

Immediately upon arriving to Taji, the TF 5-20 Infantry launched into al-Anbar province to conduct search and rescue operations in support of a downed aircraft.

In March 2007, the Regulars moved to FOB Warhorse located within the volatile Diyala Province in support of Operation Orange Justice.

In June 2007, 3-2 SBCT, with 5-20 Infantry leading the way conducted a RIP/TOA (relief in place / transfer of authority) with 3-1 HBCT and assumed responsibility for Baqubah.

This change in leadership lead to Operation Arrowhead Ripper, during which through close fighting and the employment of joint firepower, TF 5-20 Infantry routed al-Qaeda from Baqubah.

After a period of reset and re-integration, the Regulars' Attack Company and battalion headquarters were tasked with representing the United States at the annual Cooperative Spirit exercise at Hohenfels, Germany in September 2008.

After receiving their equipment and training, the task force headed north to FOB Warhorse in the Diyala Province of Iraq.

Immediately, Rock Company moved out to the Diyala Media Center Combat Outpost (COP) to begin their relief in place of the outgoing unit.

That same day the battalion suffered its first casualties of the deployment; SSG Todd Selge and SGT Jordan Shay, both of Attack Company, were killed conducting combat operations in Baqubah, Iraq.

On 11 June 2010 two more Regulars were killed conducting combat operations in Jalula, Iraq: SGT Israel O’Bryan and CPL William Yauch.