Constituted too late (5 September 1918) to even be organized and activated in time to take part in the First World War, the 321st Engineer was originally formed as a Regiment with the same numerical designation as it carries today. During World War II, it was reorganized as the 321st Engineer Combat Battalion and fought with the 96th Infantry Division in the Asiatic-Pacific Theater, earning a Presidential Unit Citation (Okinawa), two Meritorious Unit Commendations (Pacific Theater, 1944 and 1945), and a Philippine Presidential Unit Citation.
Seven years after the war’s end, the unit was given its current designation of 321st Engineer Battalion and, eleven years after that, relieved from assignment to the 96th Infantry Division in February 1963.
More than four decades elapsed before the Battalion was called into active military service over a period of several months (10 April – 14 July 2006) at its home station of Boise, Idaho; it was released from service beginning in early January 2008, and by 9 February 2008 the Battalion had completely reverted to Reserve status. During its activation, the Battalion provided support for Operation Iraqi Freedom in and around Al Anbar Province; in 2009, it was awarded a Valorous Unit Award for service there between February 2007 and 31 July 2007, in addition to a Navy Unit Commendation inscribed ANBAR PROVINCE 2006-2007.
In September 2012, the Battalion’s organic elements were inactivated at home stations as the Supply Company was concurrently constituted and activated. As of March 2023, the 321st Engineer Battalion is still stationed at Boise, Idaho and is assigned to the 301st Maneuver Enhancement Brigade.
The 311th Engineer Battalion Distinctive Unit Insignia, also called a DUI or unit crest, was first approved for the 321st Engineer Regiment, Organized Reserves on 4 July 1930 and was redesignated for the 321st Engineer Battalion on 22 September 1942. Its scarlet and red design pays tribute to the Corps of Engineers, while the pale stands for the Columbia River and the demi-sun is in recognition of the fact that the organization has been allocated to the most Western States. SEMPER TENTARE is the unit's motto, a Latin phrase that translates as "To Always Try."