ROK Marines were very good in Vietnam, probably on par with US soldiers. US commanders considered them to be the best or very close 2nd best of the allied forces in Vietnam after the Australian and New Zealand contingent.
After the Korean War, the ROK sent practically its entire officer corp to the US to study American military combat tactics. By all accounts, their American teachers were quite impressed. ROK Marines very effectively used American encirclement tactics in S. Vietnam. They were so effective, that captured Vietcong documents revealed the Vietcong units were told to avoid ROK forces at all costs unless victory was virtually guaranteed. The Americans noted that ROK officers were very meticulous in their battle plans on the one hand, but complained that they often took too long on the other hand, among other things. But then again, the ROK Marines relied heavily upon US tactical air support and air transport (via helicopter) to execute their encirclement tactics. This was the decisive advantage that all US and allied forces enjoyed in Vietnam, which the VC and NVA did not.
The Tiger Division claims a 25:1 kill ratio vs VC and NVA soldiers in S. Vietnam. The ROK Marines in general claim about a 17:1 kill ratio. By way of comparison, the US Navy seals claim a 200:1 kill ratio. The Green Berets, 101st Airborne (the "Screaming Eagles"), US Army Rangers, and so on, no doubt had some very high kill ratios too. I personally find these numbers to be both plausible and suspect at the same time. Why? Because, they all enjoyed tactical air support, not to mention air insertion and evacuation. For instance, if a platoon of Navy Seals is inserted, and they manage to kill 20 or more VC and NVA soldiers but then are subsequently surrounded by a VC/NVA regiment, they're dead meat if they can't be evacuated. Prior to such evacuations, fighter-bombers, AC-130 gunships, etc., would be called in to the rescue and pound the enemy. Thus, with out tactical air support and insertion/extraction, the entire platoon would be killed and the kill ratio would drop like a rock.
Recall the Vietminh (precursors to the NVA in the North and VC in the South) defeated the French who had 200,000 soldiers in Vietnam after WWII, until Dien Bien Phu. A number of these were the tough Legionaires which included elite Waffen SS and Werhmacht soldiers. The VC actually offered a enormous bounty for the confirmed kill of a Navy Seal, something like $25,000 if I recall correctly. They also told their soldiers to avoid the 101st Airborne, which were easily recognized by their bright white eagle patches, unless victory was all but insured. (The 101st airborne deliberately kept their bright white eagle patches as a symbol of pride, despite the US military's firm policy to keep their forces well camoflaged.)
In the 1972 inavasion by the North, it was said that the three ROK Marine division would not have been able to hold the line, since this invasion included mechanized divisions, rather than the relatively small and lightly armed VC and NVA units they typically engaged during the guerilla parts of the Vietnam War. Almost all, but not completely all, of the battles in S. Vietnam were guerilla type on the part of the VC and the NVA infiltrators.)
However, on the downside, the ROK Marines had a very bad reputation for committing attrocities, which is saying a lot since the US was quite brutal in its indescriminant bombings, "free fire zones", and so on.
Rape is a common weapon of war. However, in this respect, ROK Marines were generally quite well behaved compared to their American counterparts. There are a few isolated incidents where ROK Marines were tried, court martialed, and executed for the crime of rape by their own superiors. Unlike the US, the ROK forces had their own regulated prostitution "enterprises" in their bases. Many say this is why rape was uncommon amongst ROK Marines. (Note: I'm not trying to pick on ROK troops here. Rape is common in war, so is prostitution; study what happened on the Eastern Front between the Germans and the Russians for instance, or even the more recent Balkan crisis. Wherever the US military goes, be it in rich or poor countries, especially poor countries, invariably a thriving prostitution cottage industry pops up.)
Overall, from a purely military point of view, the ROK Marines were very good in Vietnam. They are still very good, no doubt on par with Western forces.
Edit 1: Concerning kill ratios in Vietnam, it's also highly plausible to me that it was "smarter" to count civilians caught in the crossfire as dead combatants rather than to admit otherwise and have a lot of explaining to do.
Edit 2: During the Korean War, ROK troops weren't very good. Only one, two or maybe three marine divisions were decent. These were the same ones sent to Vietnam. The N. Korean forces plowed right through S. Korean forces during the Korean War. N. Korean forces are very good for a 3rd world country, but there's no way they could replicate what they did during the Korean War. The ROK military got valuable combat experience in Vietnam. Practice makes perfect!
Edited by TranHungDao - 22-Dec-2007 at 22:21