Complex sentence: Within 15 minutes they had engaged Imboden's force and a fierce firefight ensued that lasted the entirety of the afternoon; the reinforcements, however, were not strong enough to drive off Imboden and liberate the prisoners.
Split into simpler sentences: Within 15 minutes they had engaged Imboden's force and a fierce firefight ensued that lasted the entirety of the afternoon. The reinforcements, however, were not strong enough to drive off Imboden and liberate the prisoners.