Complex sentence: A few Indians approached Sibley's camp expressing friendship but when a surgeon was killed the Indians fled behind ridges surrounding the camp and where joined by the rest of the Indian force which began firing upon the soldiers.
Split into simpler sentences: A few Indians approached Sibley's camp expressing friendship, but when a surgeon was killed, the Indians fled behind ridges surrounding the camp. There, they were joined by the rest of the Indian force, which began firing upon the soldiers.