Complex sentence: James C. Neill was promoted to lieutenant colonel during his participation in the Siege of Béxar, and ten days after the surrender of Cos at that skirmish, Houston placed him in charge of the Texian army garrison at San Antonio de Béxar.
Split into simpler sentences: James C. Neill was promoted to lieutenant colonel during his participation in the Siege of Béxar. Ten days after the surrender of Cos at that skirmish, Houston placed Neill in charge of the Texian army garrison at San Antonio de Béxar.