But their development was so largely commercial that it was only with difficulty that they could maintain navies capable of protecting their vast fleets, which were attacked by pirates, by the ships of other cities, and by each other so constantly that sea-going was a hazardous occupation, and ships perforce sailed always in convoys, or at least in the company of other ships, for protection. Then in the north William the Conqueror crossed the English Channel, defeated the Saxons at the Battle of Hastings in 1066, and the foundations for the present British Empire were laid. If the Saxons had developed a navy with which they could have met and defeated the Norman conqueror on the sea, think of the enormous difference it would have made in the history of Britain. [Illustration: A GREEK TRIREME _These warships were about 120 feet in length, and the sails and spars were taken down and sent ashore if battle was expected. The oars were operated by slaves._] During the Middle Ages following the conquest of Britain, an association of northern European cities, called the Hanseatic League, was formed in order to protect their trade, and for a time proved to be a very important factor in the maritime development of the north of Europe. Had Venice and Genoa formed such a coöperative association instead of frittering away their strength, bickering and fighting, another story would have been written in the Mediterranean. During all this time ships had been changing gradually in design. Oars still drove the fastest ships of war in the Mediterranean, but sails had taken a more important place, and now whole voyages were made by means of sails alone.