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1.
Army of the Republic of Vietnam; they are sometimes reffered to as the Southern Vietnamese Army (SVA). They were fighting against the Northern Vietnamese.
Scorned by allies and enemies alike, the Army of the Republic of Vietnam (ARVN) was one of the most maligned fighting forces in modern history. Cobbled together by U.S. advisers from the remnants of the French-inspired Vietnamese National Army, it was effectively pushed aside by the Americans in 1965. When toward the end of the war the army was compelled to reassert itself, it was too little, too late for all concerned.
by VietnamWar June 23, 2011
 
2.
Republic army of south vietnam during the vietnam war..
"you want an arvn rifle? never been fired, only been dropped once"
by The Super magnificent February 07, 2010
 
3.
(VIETNAMESE HISTORY) Army of the "Republic of Vietnam"; organization created, funded, armed, by the US government; mostly conscripts with no motivation to kill patriotic countrymen in the countryside (which is what the "Viet Cong" really were). Extremely poor fighting force; however, fairly self-confident at staging the occasional coup d'etat.

Collapsed completely in early months of 1975. Mostly behaved as one would expect a group of brutally terrorized conscripts serving a regime they despised to behave.
The other definition of ARVN includes a quote from the Stanley Kubrick movie, "Full Metal Jacket." It is used as a taunt of Vietnamese "cowardice" by young US soldiers. Of course, no sane conscript would risk his life to fight for occupiers, against an "enemy" consisting of patriotic citizens of his own country. So the taunt fell flat.
by Primus Intra Pares July 25, 2010