Complex sentence: The first two tanks he broke into would not start, then as Nelson lowered himself into the third tank, a 57-ton M60 Patton, he was finally noticed by a Guardsman, who approached the tank.
Split into simpler sentences: The first two tanks he broke into would not start. As he lowered himself into the third tank, a 57-ton M60 Patton, he was finally noticed by a Guardsman, who approached the tank.