Complex sentence: When Pepper and his men arrive, La Boeuf blows their cover by firing too soon and a firefight ensues, during which Cogburn and La Boeuf kill two of the gang, but Pepper and the rest of his men escape.
Split into simpler sentences: When Pepper and his men arrive, La Boeuf blows their cover by firing too soon. A firefight ensues, during which Cogburn and La Boeuf kill two of the gang, but Pepper and the rest of his men escape.