The plan for the 75,000–80,000 PAVN troops participating in the campaign was first to isolate Buôn Ma Thuột by cutting Highways 14, 19 and 21, precluding any South Vietnamese reinforcement. The 320th Division was then to neutralize outposts to the north and seize the Phuong Duc Airfield. The mission of the 10th Division was to then conduct the main attack on the city along Route 14. Commanding his forces from Pleiku, Major General Phạm Văn Phú, the III Corps commander, was given adequate warnings of the impending North Vietnamese attacks, but they were not given serious consideration. Phú was deceived by an elaborate North Vietnamese communications charade and his belief that PAVN movements toward Buôn Ma Thuột were diversionary operations designed to distract him from the true objective, Pleiku. The defense of Buôn Ma Thuột, therefore, was entrusted to a single Ranger group and Popular and Regional Force units (approximately 4,000 men). They were augmented at the beginning of March by 53rd Regiment of the 23rd Division. The battle for Buôn Ma Thuột began on 10 March 1975. Preceded by an intense artillery bombardment, the 10th Division quickly pushed into the city and seized the ammunition depot. That evening the 316th Division entered the battle for the city center. On the 13th, the ARVN 44th Regiment, 23rd Division and a battalion of the 21st Ranger Group were helilifted to Phuoc An, east of Buôn Ma Thuột, to form a relief force for the beleaguered city. The column, advancing into the path of thousands of refugees and military dependents fleeing the highlands, soon encountered the 10th Division. The South Vietnamese attack disintegrated and the force then retreated, joining the civilian exodus. On 18 March the PAVN had taken Phuoc An, eliminating any hope of reaching Buôn Ma Thuột. Darlac Province in its entirety then fell under North Vietnamese control. ARVN forces began to rapidly shift positions in an attempt to keep the PAVN from quickly pushing eastward to the coastal lowlands. In the final analysis, the blame for the fall of the highlands lies with General Phú, who refused to act on sound intelligence and when he realized his error, it was too late to get reinforcements to the scene. Realising the weakened state of his army, President Thiệu sent a delegation to Washington in early March to request an increase in economic and military aid. U.S. Ambassador to South Vietnam