Whether the EU commission knows what is good for it or not — always a tricky call — post-Brexit Britain should follow Roman practice in intelligently organising its borders. These were not meant to be barriers, but traversable, under Roman control.

Take the Red Sea ports. Travellers to and from Egypt were given trackable passes, at a cost, to access both the roads and the ports. Everyone understood the system, and services sprang up along the routes to keep trade flowing. The very presence of Roman soldiers created mini-markets of their own for clothes, food and sex. An inscription records the hire of a prostitute, Procla, to a military outpost for 60 drachmas plus transaction and goods levy (higher than for women in general).

Along the huge Rhine-Danube frontier, a system of passes again controlled entry into both the empire and lands beyond it. On the empire side, the military decided who was to be let in and who not, and what markets they could access. When in AD 70 a local revolt temporarily brought the system to a halt, the historian Tacitus made the tribes express their delight that they had access whenever they felt like it and did not have to pay for the privilege. Romans being Roman, that freedom did not last long.

Again, the presence of huge numbers of Roman soldiers along these frontiers was a boon to local businesses, so much so that the only way the army could be serviced was by accessing resources well beyond the frontiers. As a result, the population of towns along the Rhine-Danube was greater than in the provinces themselves. Across the empire, Romans negotiated the delicate balance between the military dominance needed to keep the province secure and the economic benefits for all in the maintenance of commerce and trade.

Eventually, of course, pressures on Germanic tribes did result in their breaking through en masse, ending the Roman empire in the West, with disastrous long-term economic consequences.