Initially, the small and poorly-equipped Cambodian Army was not up to the challenge, especially against the larger and more experienced NLF and North Vietnamese forces. However, by the summer of 1971, FANK had grown into a force of more than one-hundred thousand men with American and South Vietnamese assistance.
During the period between September 1970 and June 1971, FANK won its first victories after they successfully dislodged elements of the NLF's 9th Division along Route 13 and in some parts of the Mekong Delta.
Initially the Cambodians achieved their objective as FANK units were able to retake Barai on August 26 and Kompong Thmar on September 1. But as FANK formations were advancing towards enemy territory along Route 6, they were heavily exposed without adaquate protection from the flank. There was heavy fighting as the FANK 5th Brigade Group advanced towards Phnom Santuk while Tang Krasang was retaken on September 20. On October 5, three FANK brigades were committed to capture the areas around Phnom Santuk. The fighting there grew in intensity as the Cambodians and the NLF engaged in hand-to-hand combat. Phnom Santuk was eventually retaken, and the first phase of Chenla II was declared over on October 25 although real military success was not yet secured.
On the night of October 26, barely hours into the second phase of the operation, the North Vietnamese and NLF launched an assault on Cambodian positions along Route 6 from the Chamkar Andong rubber plantation. At the same time, the FANK 14th Battalion at Rumlong was encircled and isolated. During the following days, the 118th, 211th and 377th Battalions were forced to retreat to Tang Kauk, while the 61st Infantry Brigade pulled back to Treal, held by the 22nd Battalion.
The Cambodian army counter-attacked on October 27 but was unsuccessful and the Cambodian corridor along Route 6 was crushed by Communist troops during weeks of hard fighting. The elements of the NLF 9th Division than launched a final attack which ripped apart several battalions, causing Cambodian troops to abandon key positions on December 1. The operation was terminated two days later.
Indeed, the final attack on Cambodian positions during the month of December virtually wiped out several battalions. Militarily and psychologically the damage suffered during Operation Chenla II was a big one from which the Cambodians would never recover.