To anyone scanning the sea from the southern coast of Britain in the year 43 AD, the sight of hundreds of ships appearing one by one as dots on the horizon would have filled them with awe and dread. On a leading warship, a hundred and twenty oarsmen heaved at their work as one of the four legionary legates scanned the cliff tops expecting to see them lined with warriors as Julius Caesar had described nearly a hundred years before. Vespasian would...