PAVN forces suffered horrendous numbers of casualties from aircraft and armed helicopter attacks, artillery bombardment and small arms fire. In each instance, however, the attacks were pressed home with a professional competence and determination that both impressed and shocked those that observed them. William D. Morrow, Jr., an advisor with the ARVN Airborne Division during the incursion, was succinct in his appraisal of North Vietnamese forces – "they would have defeated any army that tried the invasion." According to the official PAVN history, by March the North Vietnamese had managed to amass three infantry divisions (2nd, 304th and 308th), the 64th Regiment of the 320th Division and two independent infantry regiments (27th and 28th), eight regiments of artillery, three engineer regiments, three tank battalions, six anti-aircraft battalions, and eight sapper battalions – approximately 35,000 troops, in the battle area.