Complex sentence: During the 1970s, the number of acquired Soviet aircraft increased to include MiG-21s and MiG-23s, and the use of the facility at Groom Lake to train new Agressor pilots was becoming more and more awkward.
Split into simpler sentences: During the 1970s, the number of acquired Soviet aircraft increased to include MiG-21s and MiG-23s. The number of aircraft and the expanded use of the facility at Groom Lake to train new Aggressor pilots was becoming more and more awkward.