The PAVN opened its campaign against Phước Long on 12 December 1974, using the 4th Battalion of the 165th Infantry Regiment to attack a South Vietnamese outpost located on Route 14, which was defended by a single South Vietnamese security company. On the same night the military zone of Bù Đốp was also attacked by a company of
Viet Cong (VC) special forces from the Phước Bình area. During the next few days the ARVN managed to recapture its positions along Route 14, but they could not clear the main road. On 13 December ARVN Colonel Nguyễn Thống Thành inspected the Route 14 area, and decided to raise the level of alertness from code 'yellow' to 'orange' for all South Vietnamese installations in the area. Due to the heavy rain which flooded the main roads leading to Phước Long, the PAVN decided to postpone further attacks which were planned to take place on 13 December. On 14 December, the 271st Regiment of the PAVN 3rd Division launched their attack on the ARVN 362nd Security Battalion, also based in Bù Đốp. After two hours of fighting nearly all members of the 2/271st Regiment had managed to penetrate the South Vietnamese defensive line around Bù Đốp, and by 11:30 the 271st Regiment successfully overwhelmed the Bù Đốp military zone. On the same day, after a night of encirclement, the PAVN 165th and 201st Regiments, both drawn from the 7th Infantry Division, established control over the Vinh Thien area and the Bù Đăng administrative centre after more than two hours of fighting. On 15 December, soldiers from the PAVN 429th Special Forces Regiment attacked Bu Na, which was defended by one ARVN company from the 363rd Security Battalion, and an artillery platoon. The 363rd Security Battalion, with strong support from RVNAF fighter-bombers based at Bien Hoa Air Base, fought back fiercely in their attempt to defend Bu Na. However, by the end of the day South Vietnamese defenders around Bu Na gave up, when reinforcements from Phước Long failed to arrive due to the closure of Route 14 by the PAVN. On 16 December, Đống flew out to Phước Long to assess the situation around the area, and to bolster the defences of Phước Long with the 2/7th Battalion, from the ARVN 5th Infantry Division. On the same day, South Vietnamese Colonel Do Cong Thanh also organised a number of counter-attacks against North Vietnamese positions around Route 14, which connects with the Bù Đăng area, but on each on occasion they were pushed back by local VC units who were guarding the area. On 22 December the ARVN 341st Security Battalion at
Bố Đức was attacked for the second time by the PAVN 165th Regiment. Subsequently, South Vietnamese units at Phước Tín, Phước Quả, and Phước Lộc were also overwhelmed by the PAVN. The ARVN attempted to stage a counter-attack on Bo Duc, but they were defeated by the PAVN 6/165th Regiment, as a result all surviving South Vietnamese units retreated back to the Bà Rá area. Two days later Đống tried to reinforce Phước Long with a regiment from the ARVN 18th Infantry Division, but President
Nguyễn Văn Thiệu overturned Đống's decision on the basis that all deployments at the regimental-level must be made by Thiệu himself. As a result, Đống had to order Brigadier general
Lê Nguyên Vỹ to reinforce the areas around Đôn Luân with units taken from the ARVN 5th Infantry Division. However, by the time regular ARVN units arrived at their destination, the PAVN had already encircled the Đôn Luân military zone, so it was within range of the formidable PAVN long-range artillery guns. Even though the ARVN received extensive air support they could only send one company into Đôn Luân, so Vỹ was forced to withdraw his forces from the area. At 05:37 on 26 December the PAVN 141st Regiment opened their attack on the ARVN base at Đôn Luân from four different directions, after 15 minutes of artillery bombardment from supporting artillery units. Due to the strong presence of PAVN artillery, ARVN artillery units at Đôn Luân were simply overwhelmed. In addition, the high calibre anti-aircraft guns used by the PAVN 20th Air-Defence Battalion also limited the effectiveness of the RVNAF air support. At 10:30 all ARVN units at Đôn Luân capitulated and Major Đặng Vũ Khoái, commander of the 352nd Security Battalion, was captured at Suoi Rat along with his junior officers. Towards the end of the day, the PAVN 141st Regiment also captured other South Vietnamese installations at Ta Be,
Phước Thiền and a helicopter base. In just two weeks the PAVN had managed to punch several holes in the defensive line of South Vietnam's III Corps, with the capture of Bù Đốp, Bù Na, Bù Đăng and Đôn Luân. For the ARVN, the loss of four important outposts in III Corps was a severe blow, especially with PAVN forces controlling all main roads leading to the district of Phước Long such as Route 14, Route 7, and the provincial roads 1 and 2. The ARVN, based at the administrative centre of Phước Bình and Phước Long, suddenly found themselves surrounded by the PAVN 4th Army Corps. ==Fall of Phước Long==