THE threat of a new war in Bosnia is so strong that “you can feel it in the air,” warns Aleksandar Vucic, the prime minister of neighbouring Serbia. It would take only a spark, he thinks, to ignite it. Some worry that the Republika Srpska, the Serb half of Bosnia, has just struck that spark.

On September 25th, the semi-autonomous region held a referendum on whether to celebrate its own national day on January 9th, the date of its founding in 1992. Bosnia’s constitutional court had declared the vote illegal, ruling that it discriminated against Bosniak Muslims and Croatians. By holding it anyway, Bosnian Serbs have struck a blow against the authority of a weak Bosnian state that has faced the threat of disintegration ever since it was created under the American-brokered Dayton accords more than two decades ago.

Milorad Dodik, the Bosnian Serb leader, has said that the Republika Srpska will vote on secession by 2018, and Sunday’s vote was seen as a trial run. The international body set up to oversee Bosnia’s 1995 peace agreement, which had the power to block the poll, did nothing, though Bosnia’s prosecution service may yet act against Mr Dodik.