ROK is a slur used to describe Koreans in a derogatory way. Troops from Korea sent to Vietnam to fight on the side of the United States. Not necessarily a derogatory term.
The word ROK is a racial slur that has been used historically to degrade and dehumanize koreans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to koreans. If you are unsure about whether or not it is appropriate to use the word ROK, it is best to err on the side of caution and avoid using it in any situation.