In July 1965, Clarke became the commanding General of the Army Engineer Center and Commandant of the United States Army Engineer School at Fort Belvoir. With the escalation of the Vietnam War, he supervised the training of engineer units for duty in Vietnam. He established a ten-month training course to prepare field officers for command at the battalion level and for duty on staffs of divisions and higher formations. Shorter courses were created to turn out platoon commanders, and he re-established the Officer Candidate School there to turn qualified enlisted personnel into junior officers. Now a major general, he was appointed the Deputy Chief of Engineers in December 1966. During his tour of duty, he was principally concerned with engineer activities in support of the war in Vietnam. On 1 August 1969, he became the Chief of Engineers, with the rank of lieutenant general. His office had an annual budget of $1.8 billion (equivalent to $ billion in ) for civil engineering works in the United States and $1 billion (equivalent to $ billion in ) for military construction. Clarke guided the Corps of Engineers as it devoted increased attention to the environmental impact of its work. He was awarded the Distinguished Service Medal and was elected to the National Academy of Engineering in 1973.
