After the war ended, Clarke was assigned to the
Manhattan Engineer District as part of a process of replacing its reservist officers with regulars. He succeeded Colonel
Franklin T. Matthias as the area engineer at the
Hanford Engineer Works in January 1946. He was responsible for the production of
plutonium there, and oversaw the town of
Richland, with a population of 25,000 people, although it was declining from its wartime peak. The Manhattan Project ended on 31 December 1946, but Clarke stayed on at Hanford as the
Atomic Energy Commission's area operations officer until September 1947. He was then transferred to
Sandia Base near
Albuquerque, New Mexico, as executive officer of the
Armed Forces Special Weapons Project at the personal request of its commander, Lieutenant General
Leslie R. Groves Jr. At Sandia he oversaw the construction of new facilities and the establishment of training programs for weapons assembly teams. He was awarded the
Commendation Ribbon for his service. In December 1949, Clarke went to
Okinawa as executive officer of the engineer district there. The base there was being expanded to counter the communist People's Republic of China, and a $500 million construction program (equivalent to $ million in ) was under way. This was accelerated after the outbreak of the
Korean War in June 1950, but much of the necessary equipment and supplies was diverted to the pressing needs of operations in the Korean peninsula. He attended a four-month course at the
Armed Forces Staff College in
Norfolk, Virginia, and then became chief of the Atomic Section of the Army's Research and Development Division, under the Assistant Chief of Staff, G-4, Lieutenant General
Williston B. Palmer, who made him his executive officer in April 1953. In February 1954, Clarke attended the three-month
Advanced Management Program at
Harvard Business School. He briefly served as head of the Construction Management Branch of G-4, where he was concerned with the funding, manufacture and emplacement of
Nike missile batteries. He then became head of the Production Mobilization Branch, with responsibility for the readiness of the national munitions and armament industries, and was special assistant to Palmer's successor, Lieutenant General
Carter B. Magruder. He attended the
National War College in 1956 and 1957. the key to the city at a White House Reception Clarke's last overseas assignment was as district engineer of the Trans-East District from 1957 to 1959. From his headquarters at
Karachi in Pakistan, he oversaw U.S. military construction in Pakistan and Saudi Arabia, and initiated transportation surveys in
East Pakistan and Burma. He oversaw $140 million worth of military construction programs (equivalent to $ million in ) in Pakistan alone. Works included
Karachi Airport and
Dhahran Airport in Saudi Arabia, and design studies for road from
Rangoon to
Mandalay in Burma, all in support of
United States Air Force (USAF) spy flights over the
Soviet Union. On returning to the United States in 1959, her served for a year as chief of staff of the United States Army Engineer Training Center at
Fort Leonard Wood, Missouri. From 1874 to 1967, the
District of Columbia was governed by three federally appointed commissioners (one a civil engineer, selected from the Army Corps of Engineers). On 1 August 1960, President
Dwight D. Eisenhower appointed Clarke as the District's Engineer Commissioner. He was the technician-in-chief, called to address the problems such as traffic gridlock, economic development and low levels of funding. At one point early in his term, he was the only commissioner available for full-time duty, because one post was vacant and the other commissioner had suffered a heart attack. In the early 1960s, he participated in talks that led to the compact agreement for construction of the
Washington Metro railway and subway system. As chairman of the District's zoning commission, he participated in early debates over the controversial proposal to build an interstate bridge near the
Three Sisters Islands in the
Potomac River, and over a planned freeway through the heart of the District. After his term as Engineer Commissioner ended on 8 July 1963, When Clarke retired from the Army on 1 July 1973 after 36 years of service, he was the last member of the West Point class of 1937 on active duty. ==Later life==