EARLIER THIS month Poland celebrated the 100th anniversary of its re-establishment as a sovereign country. From 1795 to 1918, Russia, Austria (later Austria-Hungary) and Prussia (which was absorbed into Germany in 1871) controlled the land that now constitutes Poland. The borders that used to separate those empires have vanished from world maps, but still divide the landscape. On the ground, paved Prussian roads dissolve into gravel at old border crossings. From the air, the former Habsburg and Russian territories look like a patchwork mosaic of small farming plots, whereas the west is divided into sprawling fields designed to facilitate mechanised agriculture.

Nowhere can the imperial boundaries be seen more starkly, however, than on Poland’s modern electoral map. In the country’s most recent presidential election, the east voted for Andrzej Duda, the incumbent president from the Law and Justice (PiS) party, whereas the west supported Bronislaw Komorowski of the Civic Platform (PO) party. And rather than following a gradual gradient from east to west, modern Poles’ political loyalties remain firm right up to the edge of the pre-1918 lines. The only big exceptions are in large eastern cities like Warsaw, where younger and better-heeled voters push up the liberal vote.

The stubborn persistence of these ancient borders reflects the legacy of different 19th-century development trajectories. The west formed part of a rapidly industrialising empire, and today has a dense railway network to show for it. Meanwhile, most of the east belonged to tsarist Russia, where serfdom remained legal until 1861. By 1900 incomes in what is now western Poland were five times higher than in the east. This gap remains today: Poland’s four eastern provinces are all among the EU’s poorest 20 sub-national regions. Young people growing up in the east quickly move to larger cities, seeking education and private-sector jobs. Those who feel left behind have flocked to PiS, which offers both nationalist rhetoric and monetary hand-outs.

Another potential cause of the enduring political divide was population transfers following the second world war. The Soviet Union claimed a chunk of eastern Poland as the spoils of victory, while Germany was forced to relinquish its own eastern borderlands to Poland. The Polish government responded by simply resettling millions of people from the territory it lost to the areas it gained. Separated from their families’ fields and villages, these “repatriates” developed a more open and cosmopolitan identity, and grew less receptive to fist-thumping nationalism. Meanwhile, Catholicism remained strongest in Poland’s historic eastern heartland, which developed a fiery sense of pride and suspiciousness of change.

Similar patterns can be seen elsewhere in eastern Europe. Modern Romania was also divvied up until 1918, when the northern regions belonging to the Austro-Hungarian empire were united with the south, which was ruled by the Ottoman empire until 1878. In the country’s presidential election of 2014, the north-west came out in favour of the liberal Klaus Iohannis, whereas the rest of Romania voted for Victor Ponta, the candidate of the successor party to the communist regime. One potential explanation for “Transylvanian exceptionalism” is that the region retained links throughout Communist rule with the Saxons and Hungarians who fled after 1945, and thus maintained a relative openness to the West.

Unlike in Poland, however, recent political developments are starting to obscure the Habsburg-Ottoman boundary. In 2012 Romania’s two largest parties formed an electoral alliance, which evened out some of the regional disparities among their supporters. Moreover, the same dynamic that may once have made the north distinctive is now being replicated elsewhere: the bulk of Romanian emigrants to other EU countries hail from the south and east.

Correction (November 28th 2018): This piece originally stated that modern Romania’s northern regions were reunited with the south in 1918. That should have said ‘united’, since Romania as a state had not previously existed in its current shape. This has been amended.