Complex sentence: At the height of the Cold War, a Soviet response was necessary to avoid the possibility of a new American fighter gaining a serious technological advantage over existing Soviet fighters, thus the development of a new air superiority fighter became a priority.
Split into simpler sentences: At the height of the Cold War, a Soviet response was necessary to avoid the possibility of a new American fighter gaining a serious technological advantage over existing Soviet fighters. Thus the development of a new air superiority fighter became a priority.