A brief history of borders
Most of us take it for granted that every person on earth is the citizen of a nation state, but this is a relatively recent concept.
Take Europe for example. Although there had long been empires that stretched across large tracts of land, up until the Middle Ages Europe was essentially made up of multiple city states. Indeed, the modern day passport is believed to have begun as a medieval document required to pass through the gate ("porte") of a city wall. In general, documents were not required when arriving at sea ports, which were considered open trading points.
It was not until the 15th century that the concept of a national border came into being - triggered, in part, by the Hundred Years War (1337-1453), which started as a tussle between two royal houses for the French throne and ended with both France and England embracing a newly discovered sense of nationalism.
For much of the rest of the world, national borders did not follow ...