Complex sentence: The Texian army also moved east, but to a Brazos River training site, causing many to erroneously believe Houston was running away and giving Santa Anna a false sense of complacency.
Split into simpler sentences: The Texian army also moved east, but to a Brazos River training site, causing many to erroneously believe Houston was running away. Texian troop movements gave Santa Anna a false sense of complacency.