Complex sentence: Concerned by Soviet postwar activities in the Arctic, the U.S. Air Force initiated B-29 reconnaissance flights over the Arctic region beginning in 1946, and by 1951, the reconnaissance trips to the North Pole were being implemented on daily basis.
Split into simpler sentences: Concerned by Soviet postwar activities in the Arctic, the U.S. Air Force initiated B-29 reconnaissance flights over the Arctic region beginning in 1946. By 1951, the reconnaissance trips to the North Pole were being implemented on daily basis.