Complex sentence: Following the defeat at San Juan de Ulúa, Drake vowed to get revenge and thus made two minor voyages to the West Indies, in 1570 and 1571, of which little is known.
Split into simpler sentences: Following the defeat at San Juan de Ulúa, Drake vowed revenge. He made two voyages to the West Indies, in 1570 and 1571, of which little is known.