Last week's vote to leave the European Union was hardly the United Kingdom's first Brexit. More than a few countries in the Middle East, Asia and Africa have scars to show from Britain's long and haphazard withdrawal from its onetime empire. Borders drawn through Palestine and around Iraq continue to fuel bitter conflicts throughout the region and beyond. The welter of tribes compacted into modern Burma, now Myanmar, battle still for greater autonomy. South Sudan, clubbed together with its larger neighbor for half a century, is struggling violently to find its independent footing.

The first in this ill-starred series of colonial pullouts proved almost unimaginably brutal. Up to a million people may have died in the sectarian massacres that followed the end of the British Raj in August 1947. Some 14 million people were uprooted from their homes and forced to flee across the new borders dividing India and Pakistan. That searing experience gave birth to one of the world's fiercest national rivalries and its most dangerous nuclear standoff.

The red that traders saw bleeding across their screens last week hardly compares, of course. The new boundary Britain has drawn — or more properly, redrawn — between itself and Europe isn't likely to contribute to generations of misery, let alone spark a war. One critical mistake, however, does link Britain's world-striding imperial proconsuls to today's insular Brexiteers. Both seem to have imagined that borders themselves are uncomplicated things — that the act of drawing a line can separate you and your rules from me and mine.