President Donald Trump’s summit with President Vladimir Putin on July 16 will take him to Helsinki, one of Russia’s many lost possessions. From Finland to Mongolia, the Russian Empire and then, in somewhat different borders, the Soviet Union once ruled more than a sixth of the planet’s surface. Mr. Putin has famously described the loss of this empire, which happened nearly overnight in December 1991, as the “greatest geopolitical catastrophe” of the 20th century—not least because it has stranded tens of millions of Russian-speakers beyond Russia’s shrunken frontiers.

The...