Winter-Spring 1970, 1 November 1969-30 April 1970. An increase in enemy-initiated attacks, at the highest level since 4-5 September signaled the start of the first phase of the Communist winter campaign. This was highlighted by intensified harassment incidents, and attacks throughout the Republic of Vietnam. In November-December these were heaviest in Corps Tactical Zones III and IV (around Saigon), primarily directed against Vietnamese military installations in order to disrupt the pacification program. The most significant enemy activity occurred in November with heavy attacks upon By Prang and Duc Lap in CTZ II (Central Vietnam). By February 1970 the focus of enemy activity began to shift to CTZ I and II. Attacks increased steadily, reaching a peak in April 1970. Hostile forces staged their heaviest attacks in the Central Highlands near Civilian Irregular Defense Group camps at Dak Seang, Dak Pek, and Ben Het in I CTZ. The enemy also conducted numerous attacks by fire and several sapper attacks against U.S. fire support bases. This high level of enemy activity began in I CTZ in April and continued through May. During the period 1 November 1969 through 30 April 1970 U.S. and allied forces concentrated on aggressive operations to find and destroy enemy main and local forces, the penetration of base camps and installations and the seizure of enemy supplies and materiel. These operations sought to deny the enemy the initiative and to inflict heavy losses in men and materiel. Further progress was made in Vietnamization through improving the Republic of Vietnam Armed Forces. As a result of these advances three brigades of the 1st U.S. Infantry Division and several major U.S.M.C. units were withdrawn from Vietnam during this period. The enemy made several efforts to take the offensive at Dak Seang, which was attacked on 1 April 1970 and remained under siege throughout the month, and at Quang Duc in the By Prong-Duc Lap area which ended on 28 December. Only Vietnamese forces were engaged in both of these operations, the Quang Duc campaign involving some 12,000 ARVN troops. South Vietnamese forces again took the offensive on 14 April in a bold 3-day operation in the Angel's Wing area along the Cambodian border. The Vietnamese Army completed this mission in an aggressive professional manner without U.S. support-further evidence of their growing proficiency.