VIENNA—As I look at “The Kiss,” Austrian painter Gustav Klimt’s delicate masterpiece, in one of Vienna’s Belvedere palaces, my head fills with the memories of my two trips to war-torn Bosnia in the 1990s. The papers recently have been full of coverage about the capture of Radovan Karadzic, the former president of Bosnia’s breakaway Serbs. He is believed to have spent part of the last year hiding in Vienna—the city from where Bosnia was governed until the collapse of the Austro-Hungarian Empire at the end of World War I.

How could the same Bosnia and Herzegovina that began the 20th century as almost a model of multi-confessional, pluralist coexistence become a byword for tribal brutality at the end of it? If we are to make the 21st century something better than the last one, then the lessons of the Bosnian war—including the capture of Karadzic and his extradition to The Hague—should serve as our reminder.

Although nationalism is an extended form of tribalism, its worst exponents are often a country’s more cultivated people. That Karadzic was an average psychiatrist and a mediocre poet does not detract from the fact that he was also well-read, that he had studied abroad (including a spell in the United States) and that his father had fought against Nazism and communism. He defended his conduct of the war, centered on ethnic cleansing, with elaborate sophistry. My brother, who negotiated humanitarian relief efforts with him on several occasions, was amazed by how convinced this genocidal nationalist was that he was carrying out a war of liberation.

Karadzic reminded us for the umpteenth time that, in politics, means matter more than ends. He would not have been at war with Bosnia’s Muslims and Croats, he used to say, if the Bosnian government had not declared its independence from Yugoslavia in 1992. But the dozens of thousands of Bosnians killed and the hundreds of thousands forced out of their homes by his army were the negation of his arguments. Even if the sequence of events supported his logic, his methods proved that his was not a war of defense but aggression based on ethnic cleansing. One has only to read his chilling directive before the massacre of Srebrenica to see this beyond doubt.

I visited two cities under Serbian siege during the conflict: Bihac, the Muslim enclave in northwestern Bosnia, and Sarajevo, the capital. Those were not cities at war—they were cities dying a slow death. I remember seeing hundreds of people critically wounded by the incessant shelling and the sniper fire: Not one of them was a soldier.

All sides in the former Yugoslavia bear responsibility for the war. They were all too quick to break away without taking into account the sensitivities and fears of their minority populations. And all sides committed atrocities under orders from ideologues. But just as the former leader of the Bosnian Serbs cannot be judged by his purported goal—the preservation of Yugoslavia in the interest of Bosnia’s Serbian minority—he cannot be excused because of his enemies’ actions. History is replete with cadavers from crimes committed by fanatics whose stated aims seemed just and whose enemies were in the wrong. On a lesser scale, was John Brown any less guilty of practicing terrorism in 19th-century America because he killed, robbed and tyrannized those who came under his control in the name of the abolitionist cause?

In one respect, Karadzic won the Bosnian war. Under the Dayton accords of 1995, Bosnia and Herzegovina was turned into a loose federation made up of a Muslim-Croat coalition and the Serb entity much along the lines of his breakaway republic. The efforts made by the international community to centralize power at the top of the federation all these years have generated resentment among non-Muslims. The tensions could one day degenerate into violence.

Curiously, of the two entities Republika Srpska has been by far the more successful one. Its market-friendly policies have turned it into a more prosperous society than the Muslim-Croat coalition, which is one huge welfare program supported and directed by the international community through a high representative who calls the shots at the federal level. Many businesses in Bosnia and Herzegovina have moved from the Muslim-Croat sector to the Serbian entity.

Karadzic’s trial will probably not throw light on these new developments. But let us hope that it will at least tell all those young people for whom the Bosnian war doesn’t mean anything how we got here.