Bombing of North Vietnam (Operation Rolling Thunder) had ended in 1968, and as a result, North Vietnamese forces had built up their air defenses and continued to pour men and equipment into the South via the Ho Chi Minh Trail. By the beginning of 1972 there were only about 235 USAF tactical combat aircraft in Southeast Asia. Vietnamization was severely tested by the Easter Offensive of 1972, a massive conventional invasion of South Vietnam by North Vietnamese Army forces in spring 1972. On 30 March 1972, the People's Army of Vietnam (PAVN) launched an all out invasion of South Vietnam with over 13 divisions, pushing South Vietnamese units aside with little difficulty. President Nixon stepped up air strikes to turn back the invasion, or at least to slow it down.