Bloomington, Ind. — An old train route south from the eastern Polish city of Przemysl passes through Ukrainian territory, then back into Poland. The tracks are a relic of the prewar past, when this was all Polish territory, before the Soviet Union “liberated” western Ukraine in 1939 from Poland and incorporated it into the Ukrainian Soviet Socialist Republic.

In the Communist period, some traveled this loop — no stops allowed — only to get a glimpse of a land then forbidden and inaccessible. But in 1980 and 1981 the brief Soviet excursion became a propaganda jaunt for Poles. Passengers flashed Solidarity signs from opened windows, boasting to their neighbors of the dissident trade union whose rapid growth threatened the Communist monopoly on power in Poland.

Before long, the Soviet regime insisted windows be kept shut on their territory, and eventually their Polish comrades suspended the route. A decade later, his country and Communist rule were gone, and Poles and Ukrainians could get closer than through the grimy window of a lumbering passenger train. But the emotions of that old train route still illuminate the fears and hopes that now make Poland a central player in the sharpest European crisis since 1989.

For the last quarter-century — the first time in modern history — Poles have not faced an existential threat from the East. But within living memory, Poland lost its eastern provinces when Hitler and Stalin carved it up in 1939; in 1945, the loss became permanent in a redrawn Poland that now included former German lands. So invasions, dismemberments and wholesale remappings of nations are not implausible to Poles. The idea that Vladimir V. Putin, Russia’s president, could simply send his troops to occupy and effectively annex territory from Ukraine without real provocation may have seemed fantastical from farther away, but not from Warsaw....