Croatian Prime Minister Andrej Plenković, speaking on HRT, has reopened a discussion many in the Balkans assumed was closed two decades ago. Talking about relations with Serbia, he listed the „residual questions" - war crimes, missing persons, and resolving the question of the border between the two countries.

The sentence isn't the problem; the logic behind it is. Croatia's cadastral model of the Danube border does not align with the international standard of the „median line." If the Croatian model were applied, parts of the Serbian towns of Apatin, Sombor and Bačka Palanka would fall to Croatia - literally, as a 21st-century cadastral intervention.

Serbia hasn't officially reacted yet. But in a country that has spent 25 years building a state on the premise of „borders that don't change," these words from Zagreb will be read as an announcement. And the question no Balkan prime minister wants to answer - at what point do polite „residual questions" turn into active ones, and what happens when the EU demands a resolution?

This isn't a new discussion. Similar tensions between Croatia and Bosnia and Herzegovina, between Kosovo and Serbia, between Bulgaria and Macedonia, are all run on the same cold tone - that every „inherited question" has its own time to be opened. That time is always chosen by the bigger players, and rarely overlaps with the interests of the people who actually live on those borders.