“Europe is worried these days about the rise of the far right, but this place was way ahead of the curve in showing how dangerous and enduring ethno-nationalism can be,” said Tim Clancy, an American resident of Bosnia who worked in Mostar throughout the war helping victims of the fighting.

Both of Mostar’s fire brigades are part of the same municipal fire service — just as Mostar’s two garbage collection companies, two hospitals, two electricity companies, two bus stations, two popular nightclubs and two soccer teams all technically serve the same city. But they are in reality barricaded behind the zigzagging line drawn in blood a quarter of a century ago, during Europe’s worst conflict since World War II.

“Everything in this town is very clear. Everyone knows whose territory lies where,” Mr. Golos said, adding that his brigade never gets asked by Mostar’s fire service dispatcher to fight a blaze in an area that was controlled by Croat forces during the war, no matter which of the city’s two fire stations is closer.