Complex sentence: Fannin also believed that by occupying Goliad, he could prevent Mexican commander Antonio López de Santa Anna from drawing supplies from the Gulf of Mexico, but Fannin was called to assist Colonel William Travis at the Alamo.
Split into simpler sentences: Fannin also believed that by occupying Goliad, he could prevent Mexican commander Antonio López de Santa Anna from drawing supplies from the Gulf of Mexico. But,, Fannin was called to assist Colonel William Travis at the Alamo.