Before this policy change took place another one, at a still higher command level, had occurred. On 23 June, FAF planes struck at eight North Korean hydroelectric plants in the central and northwestern part of the country. The attack represented a departure from the intense interdiction of enemy lines of communication (Operation STRANGLE) which, since May 1951, had characterized FAF support operations. The shift came about after a Far East Air Forces study on the effectiveness of the interdiction campaign had concluded, in part, that the program had been indecisive.[89] [89] HistDiv; Air Univ, USAF, _United States Air Force Operations in the Korean Conflict, 1 November 1950–30 June 1952_, USAF Hist Study No. 72 (Washington, 1955), p. 159, hereafter USAF, _Ops in Korea_, with appropriate number. The Air Force operations were published in three books, numbered 72, 73, and 127. For more than a year preceding the 23 June attack, the Fifth Air Force had concentrated its ground support efforts on the disruption of Communist communication lines so that the enemy would be unable “to contain a determined offensive ... or to mount a sustained offensive himself.”[90] During the lifetime of the doctrine, no major offensive had been launched by the enemy, and on this fact was based the claim for success of the interdiction program. Opponents, however, pointed out that despite this maximum FAF air effort, the Communists had built up their strength, including support areas immediately to the rear of their front lines and resupply installations. As the recent UN commander, General Matthew B. Ridgway,[91] told members of the Senate Committee on Armed Services on 21 May 1952, the same month that FAF had begun to shift its air effort away from interdiction, “I think that the hostile forces opposing the Eighth Army ... have a substantially greater offensive potential than at any time in the past....”[92] [90] Futrell, _USAF, Korea_, pp. 435–436. [91] General Mark W. Clark had succeeded Ridgway as UN Commander on 12 May 1952. Ridgway was to take over as the new Supreme Allied Commander, Europe, 1 June, replacing General of the Army Dwight D. Eisenhower, who was returning to the United States. [92] Cited in Futrell, _USAF, Korea_, p. 435.