Operation Chenla II was launched on 20 August 1971, again catching the PAVN/VC by surprise. Initially, the FANK task-force commanded by Brigadier-General Hou Hang Sin achieved their objective, as the FANK were able to retake Barai on 26 August and Kompong Thmar on 1 September. But as FANK formations were advancing towards PAVN/VC-held territory along Route 6, they were heavily exposed to attacks without adequate protection from their flanks. There was heavy fighting as the FANK 5th Brigade Group advanced towards Phnom Santuk while Tang Krasang was retaken on 20 September. On 5 October, three FANK brigades were committed to capture the areas around Phnom Santuk. The fighting there grew in intensity as the Cambodians and the PAVN engaged in heavy hand-to-hand combat. Phnom Santuk was eventually retaken, and the first phase of Chenla II was declared officially concluded on 25 October, although real military success had not yet been secured.
