Trade and the Military. The responsibility for the ease of travel in the ancient world was shared by the twin incentives of profit and security. Harbors such as Ostia and outposts such as Dura-Europos teemed with merchants hocking their wares and with soldiers maintaining their emperor's brand of order. Military and administrative travel included more than the movement of troops: ambassadors, sometimes with significant entourages, had to visit separate kingdoms and empires; news and information had to move efficiently among leaders. Commercial travel was also multifaceted: the archaeological evidence of shipwrecks and storefronts show all varieties of goods moving through desert caravans or coastal convoys. Soldiers and tradesmen made up a large portion of the total population of the empire, and it is fair to assume that from them alone unprecedented numbers of individuals were broadening their horizons: encountering new gods...