The anthem was originally adopted in 1941 and reconfirmed in 1947, around the time of independence from France. In 1970, the monarchy was abolished, thereby replacing the anthem as well. After the communist victory in 1975, former royalist symbols, including “Nokoreach”, were reinstated for a short while until replaced with their own national symbols. During exile, the second verse of Nokoreach was declared to be the anthem of Cambodia[1]; after the royalist forces defeated the Vietnamese puppet regime in 1993, putting an end to their long civil war, the royalist anthem (all verses) was also restored to Cambodia. The title of the anthem is derived from the name of an ancient Khmer kingdom.
Sources:
[1] Siegfried Ehrmann, “A Flag With A Tortured Past: The Khmer Rouge Flag,” mekong.net, accessed February 25, 2023, https://www.mekong.net/cambodia/khmer_rouge_flag.htm .
Special thanks to: Suttipong Phuensaen for some of this information.