MiG-15 fighter-interceptors, and SAC's 27th Fighter Escort Wing began escorting the bombers with F–84 Thunderjets. Ground-directed bombing (GDB) was subsequently used for close air support (CAS) missions after three SAC radar bomb scoring (RBS) squadron detachments (Dets C, K, & N) arrived at Pusan in September 1950. In 1951, SAC "began to eliminate its combat groups", transferring medium bombardment groups "to Far East Air Forces (FEAF) Bomber Command for combat." In 1951, LeMay convinced the Air Staff to allow SAC to approve nuclear targets, and he continued refusing to submit war plans for JCS review, which the JCS eventually came to accept (of 20,000 candidates in 1960, SAC designated 3,560 as bombing targets—mostly Soviet air defense: airfields and suspected missile sites.) Although experimented with prior to World War II, SAC refined aerial refueling to a fine art. SAC's in-flight refueling mission began in July 1952 when its 31st Fighter-Escort Wing refueled sixty F-84G Thunderjets from Turner AFB, Georgia to Travis AFB, California non-stop with fuel from twenty-four KB-29P Superfortresses modified into aerial tankers. Exercise FOX PETER ONE followed with 31st FEW fighters being refueled Hickam AFB en route to Hawaii. On 15 March 1953, a 38th Strategic Reconnaissance Squadron RB-50 returned fire on a Soviet MiG-15, while a 343d Strategic Reconnaissance Squadron RB-50 was shot down over the Sea of Japan 2 days after the Korean Armistice, while on 7 November 1954, an RB-29 was shot down near Hokkaido Island in northern Japan. By the time of 27 July 1953 Korean War cease-fire, SAC B-29s had flown over 21,000 sorties and dropped nearly 167,000 tons of bombs, with thirty-four B-29s lost in combat and forty-eight B-29s were lost to damage or crashes. Cold War and massive retaliation SAC's first jet strategic bomber was the swept-wing B-47 medium bomber, which first entered service in 1951 and became operational within SAC in 1953. The B-47 was a component of the October 1953 New Look strategy, which articulated, in part, that: "...to minimize the threat...the major purpose of air defense was not to shoot down enemy bombers—it was to allow SAC...to get into the air[--and] not be destroyed on the ground[--to allow] massive retaliation.". Concern of a bomber gap grew after the 1955 Soviet Aviation Day and the Soviets rejected the "Open Skies" Treaty proposed at the Geneva Summit on 21 July 1955. US bomber strength peaked with "over 2,500 bombers" after production