Operation Hardihood was a security operation conducted from 16 May to 8 June 1966 during the Vietnam War by the U.S. 503rd Infantry Regiment, the 1st Battalion, Royal Australian Regiment (1RAR) and the 5th Battalion, Royal Australian Regiment in Phước Tuy Province, South Vietnam to secure the area around Nui Dat for the establishment of a base area for the 1st Australian Task Force.

The 5th Battalion, Royal Australian Regiment is a regular motorised infantry battalion of the Australian Army. Originally established in 1965 it would serve two tours of South Vietnam before it was linked with the 7th Battalion to form the 5th/7th Battalion, Royal Australian Regiment in 1973. In late 2006 the two units were de-linked, and 5 RAR again joined the Australian Army's order of battle in its own right. It has since served in Iraq, East Timor and Afghanistan.