1506-1785 - Historic Spanish military flag. The Spanish Cross flag (Cross of Burgundy dating from the late Middle Ages) has the serrated or roughened red saltire of St. Andrew centered on a white field. The flag was used by the Spanish crown and military. St. Andrew was the patron Saint of Burgundy and the emblem was introduced to Spain through marriage between the Duke of Burgundy and the daughter of King Ferdinand and Elizabeth of Castile and Aragon. Current versions of the flag continue to fly over the US Territory of Puerto Rico at the National Parks historic Spanish fortresses of Castillo San Cristo'bal and El Morro in old San Juan.