Belgae and Celts, known as Gauls, invaded Normandy in successive waves from the 4th to the 3rd centuries BC. Much of our knowledge about this group comes from Julius Caesar's "de Bello Gallico". Caesar identified several different groups among the Belgae who occupied separate regions and lived in enclosed agrarian towns. In 57 BC, the Gauls united under Vercingetorix in an attempt to resist the onslaught of Caesar's army. Even after their defeat at Alesia, the people of Normandy continued to fight until 51 BC, the year Caesar completed his conquest of Gaul.