Complex sentence: In the Battle of Mier that resulted, although the Texans, who were outnumbered ten to one, inflicted heavy casualties on the Mexicans--650 dead and 200 wounded--they were forced to surrender on December 26.
Split into simpler sentences: In the Battle of Mier that resulted, the Texans were outnumbered ten to one. They inflicted heavy casualties on the Mexicans--650 dead and 200 wounded--but they were forced to surrender on December 26.