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the enemy to backlog supplies in Cambodia, increasing their already large depots. It was apparent that there could be no better time to strike at the heart of the enemy's supply base.
This area of Cambodia that borders III Corps Tactical Zone of the Republic of Vietnam had been used extensively by the Viet Cong and North Vietnamese since 1961 after the organization of Central Office South Vietnam, the headquarters commanding all Viet Cong and North Vietnam Army forces in III and IV Corps. During this period of nine years, the enemy had developed a series of base camps to house the administrative units of the Central Office South Vietnam headquarters plus a large number of associated units and subordinate commands. This included an artillery command, a security guard regiment, the 5th Viet Cong Division, the 7th North Vietnam Army Division, the 9th Viet Cong Division, and many service regiments. Although Allied forces had approached the border in large operations such as JUNCTION CITY, ATTLEBORO, and more recently the operations of the 1st Cavalry Division, the enemy had always had the advantage of being able to withdraw to the safety of his Cambodian sanctuaries.
The "Fishhook" of Cambodia extended from the generally flat plains adjacent to Mimot (Map 9) northeast through roughly rolling plains; and east to the roughly dissected hills and low mountains near O'Rang. Multi-canopied, dense undergrowth forest was the dominant natural vegetation throughout the area. Rubber plantations were found primarily in the western section. Open areas with dry crops, upland rice or marshes were scattered throughout. This area, because of its inaccessibility and the dense vegetation cover was extremely favorable to guerrilla warfare and restricted heliborne and mechanized operations. Generally, concealment from both aerial and ground observations was excellent.
The sequence of events which unfolded during the weeks which preceded the operation—the removal of Prince Norodom Sihanouk as Cambodian Chief of State, the closing of the port of Sihanoukville, and limited allied operations in other portions of Cambodia—had alerted the enemy to possible cross-border operations. Nevertheless, the enemy was not prepared for the massive assault to follow, nor was he prepared for an assault in depth. It is clear now that the North Vietnam Army felt that any crossing of the border would be a shallow operation of perhaps two or three kilometers. He was to discover, to his consternation, that the allied forces were at times twenty kilometers to his rear.
On 26 April 1970 Major General Elvy B. Roberts, the Division Commander, received instructions from Lieutenant Gen[eral]