Complex sentence: By the end of 1971, the Gnat proved to be a frustrating opponent for the technically superior Sabres and had lived up to its Indian Air Force nickname of "Sabre Slayers" since all its combat "kills" during the two wars were against Sabres.
Split into simpler sentences: By the end of 1971, the Gnat proved to be a frustrating opponent for the technically superior Sabres. The Gnat was referred to as "Sabre Slayers" by the Indian Air Force since all its combat "kills" during the two wars were against Sabres.