BANJA LUKA, Bosnia and Herzegovina — When Bosnia’s medicines agency inspected oxygen sold to hospitals to treat Covid-19 patients in the country’s Serb-controlled region in September, it made a shocking discovery: The oxygen was meant for use only in industrial machines, not on human beings.

But rather than trying to correct the situation, the region’s leader, Milorad Dodik, a pugnacious Serb nationalist, instead tried to tear apart the multiethnic fabric of the Bosnian state, a fragile union created in 1995 by American diplomacy out of the wreckage of war.

First, Mr. Dodik announced that he was creating his own medicines agency and withdrawing his fief, which covers roughly half of Bosnia’s territory, from the oversight of central government inspectors.

Since then, he has threatened to pull out of Bosnia’s multiethnic armed forces and form his own, exclusively Serb army. He also wants out of the state’s tax agency, its intelligence service and its judiciary, vowing to accelerate what he calls the “peaceful dissolution” of a Bosnian state birthed by a peace deal forged in 1995 in Dayton, Ohio.

Bosnian investigators have traced the oxygen shipments to a company controlled by one of Mr. Dodik’s close political allies, and some foreign diplomats say his secessionist threats are a means to deflect from allegations of corruption.

But in a region where the shadow of war is everywhere, many Bosnians fear that the country’s peace is under threat.

“It will not be peaceful,” warned Sefik Dzaferovic, one of Bosnia’s three presidents, each elected to represent a particular ethnic group.

A patchwork of different peoples and religions, Bosnia has long been a tinderbox for larger conflagrations.

It was in Sarajevo, Bosnia’s capital, that a teenage Serb nationalist set off World War I by assassinating an Austrian archduke in June 1914, and where the seemingly deranged rants of a Serb psychiatrist, Radovan Karadzic, presaged a three-year spree of bloodletting in the 1990s. Those Balkan wars left roughly 140,000 people dead, drew in NATO warplanes and soldiers and created a rift between Russia and the West that remains today.

Now the United States and the European Union, which Bosnia aspires to join, are desperate to stop the new crisis from escalating into conflict, or creating the sort of political instability that Russia could exploit. Russia, which wants to prevent Bosnia from joining the bloc or NATO, is already siding with Mr. Dodik.

The frictions in Bosnia are rooted in the 1995 Dayton peace agreement, brokered by the United States. The deal stopped the fighting but created an elaborate and highly dysfunctional political system, with a weak central authority in which different ethnic groups share power. Two of the country’s presidents include Mr. Dodik, who represents Serbs, and Mr. Dzaferovic, who represents Bosnian Muslims, known as Bosniaks.

Mr. Dodik has made noises about Serb secession for more than a decade but has never before prompted such a volatile crisis. A report in October by the United Nations’ senior official in Bosnia, Christian Schmidt of Germany, described the situation as “the greatest existential threat” to the country’s survival since the early 1990s.

Mr. Schmidt, in a recent interview, played down the risk of a return to bloodletting and said he expected Mr. Dodik to back off his threat to form a separate ethnic Serb army.

Among many Bosnians, however, fear is again on the march.

When Mr. Schmidt met in mid-December with students at a vocational school in Tuzla, a town where Bosnia’s different ethnic groups have tended to live in rare harmony, he was repeatedly asked what he was doing to prevent a return to war.

One student recalled that his parents had lived through the horror of Bosnia’s 1992-1995 conflict and asked, “Can you promise us that this won’t happen again?” Another told Mr. Schmidt, “I can’t wait to leave this country where the word ‘war’ is being used more and more.”

A teacher displayed a photograph from 1991 that showed a dozen of her male students at the time, all looking relaxed and happy. A quarter of them, she said, were killed in the fighting that began shortly after.

In Europe, the response to Mr. Dodik’s provocations has been mixed. Germany and Britain are discussing sanctions. But Hungary’s authoritarian leader, Viktor Orban, recently visited the Serb region’s capital, Banja Luka, to offer support to Mr. Dodik and has vowed to veto any move by the European Union to impose sanctions.

Under the Dayton settlement, Bosnia is divided into two largely self-governing parts: Mr. Dodik’s Serb territory, known as Republika Srpska, and a federation controlled by Bosniaks and ethnic Croats. The federation, in turn, is divided into 10 “cantons,” each with its own government.

Many Bosniaks view Mr. Dodik’s disruptive actions as proof that Bosnian Serbs should never have been allowed by the Dayton deal to hang on to their own domain, an entity midwifed by men like Mr. Karadzic and the Bosnian Serb former commander Ratko Mladic, who have since been convicted of genocide at The Hague for the Srebrenica massacre in July 1995 and other atrocities.

But Mr. Dodik and many of his fellow Serbs still deny war crimes committed by ethnic kin and instead see themselves as victims, as they did during the war. They now claim that Bosnian Serbs are being unfairly picked on, after a decision in July by Mr. Schmidt’s predecessor as U.N. envoy that outlawed the denial of genocide. The ban applies to all ethnic groups, but many Bosnian Serbs see it as targeted at them.

Mirko Sarovic, the leader of a Serb political party opposed to Mr. Dodik, denounced the ban as a “huge mistake.” In an interview, he said it had emboldened belligerent nationalists, bolstered Mr. Dodik’s previously waning public support and encouraged him to embark on a “reckless adventure” that “has no chance of succeeding and has huge potential to provoke conflict.”

Mr. Dodik is a onetime American protégé whom the Clinton administration praised in 1998 as a “breath of fresh air.” Now, President Biden’s special envoy to the region, Gabriel Escobar, calls him a menace who “stabs the heart, strikes the heart of Dayton.”

In October, Mr. Dodik warned that Serbs in Bosnia would “defend ourselves with our forces if necessary” and said that “our friends” — namely Russia and nearby Serbia — could be counted on to repel any effort to rein him in by the NATO alliance.

Serbia, however, has shown no interest in repeating its role in the 1990s, when it sent weapons and paramilitary gangs to support ethnic kin in Bosnia. And how much Russia really supports Mr. Dodik is unclear.

Last month, he returned from a visit to Moscow claiming that he had received pledges of support during a meeting with President Vladimir V. Putin. But the Kremlin, which usually announces meetings well in advance, waited days before confirming it had happened. Then it put the Serb leader in his place, saying that Mr. Putin’s “main event” that day had been a “coal conference,” not Mr. Dodik.

Yet the Kremlin clearly delights in seeing Bosnia in disarray, given that the United States and Europe once championed it as a showcase of successful nation-building. For years, Mr. Putin has warned the formerly communist lands of Eastern Europe that Western promises of peace and prosperity are hollow.

The big question is whether Mr. Dodik’s threats are real or are mostly political theater to rally his nationalist base ahead of elections in October.

“He probably doesn’t really know himself where this all leads,” said Mr. Schmidt, the U.N. envoy, adding that he was on a “dangerous and slippery road.”

Mr. Dodik has indicated in private, diplomats say, that his primary interest is keeping state prosecutors out of his domain so as to eliminate the risk that credible reports of rampant corruption ever get seriously investigated — including the scandal over industrial oxygen for Covid patients.

The Bosnian health investigator traced those shipments to a company based in Mr. Dodik’s hometown, Laktasi, and controlled by a close political ally.

“It is all a political game, and politics in Bosnia is just a smoke screen to cover up crime,” Aleksandar Zolak, the medicines agency’s chief, said in an interview. “Dodik knows that he can only be exposed by independent institutions and people who tell the truth, so is doing everything he can to destroy them.”

Mr. Dodik, though, has used the moment to grind the central government to a halt. The three presidents are supposed to meet every two weeks to sign off on key proposals. But they have not met since October, when Mr. Dodik showed up with an accordion and started singing Serb folk songs in his office with a group of supporters.

Since then, he has rejected or ignored all proposals put before him and his fellow presidents.

“Dodik survives on conflict,” said Branislav Borenovic, a Serb opposition party leader. “He hates stability because he then has to explain why we are living like we do,” he said, adding that Mr. Dodik “plays on the emotions of his people and doesn’t care about the consequences.”

Even if Mr. Dodik is just playing politics, Mr. Borenovic said, his antics are stirring passions to a dangerous pitch: “In a country of three million people, you can always find a few idiots to light the fire.”