WHEN Bosnian Serbs launched a major offensive last October against Tuzla, its poorly armed Bosnian Army defenders took a drastic step to stem the onslaught.
They deployed tanker trucks containing massive quantities of chlorine along the front lines and threatened to detonate them, unleashing a storm of poison gas that would have swept across the region and into Serbia and Croatia.
The Bosnian Serb attacks stopped, and Tuzla has seldom been fired on since. The closest front now is about 10 miles away.
Tuzla's terrible weapon is a testament to its rare resolve. Amid the ruination of war in Bosnia-Herzegovina, the city has achieved unparalleled economic success, keeping in operation about 30 percent of one of the largest concentrations of heavy industry in former Yugoslavia.
But its greatest achievement has been the successful struggle, so far, against the communal hatreds that sundered Yugoslavia and ignited the "ethnic cleansing" of other parts of Bosnia.
Since the Serb-dominated former Yugoslav Army and about half of Tuzla's 30,000 Serbs withdrew last summer, the city has maintained peace among its Muslim majority, Croats, and remaining Serb residents.
"Many people believe that we in Tuzla have created the possible out of the impossible," says Mayor Selim Beslagic, a Muslim Slav, sitting in his office and clad in camouflage combat fatigues.
But as Bosnian Serbs overrun Muslim enclaves in the eastern part of the republic, forcing refugees to be evacuated here, Tuzla's ethnic and economic seam is showing signs of strain.
Mayor Beslagic's government - the only one in any major city of former Yugoslavia run by the Alliance of Reformist Forces, a virtually defunct anti-nationalist coalition - gets the most credit for Tuzla's success. Economic vitality
Coal mines provide fuel for an electricity station that supplies 200 megawatts to factories, including a chlorine plant, and about 1 million residents and refugees in central Bosnia, the biggest region still loyal to the Muslim-led Bosnian government.
The station is about to increase its output by 200 megawatts, most of which will be sold to Croatia for hard cash. Tuzla also mines salt and produces detergent, flour, and munitions.
Officials estimate that 45 percent of the 120,000 residents are employed in local government, the Army, private enterprises, and industry, earning a monthly average of 10 German marks.
Pittances by outside standards, those statistics are unknown elsewhere in Bosnia.
All of this has been managed even though Tuzla was cut off by Bosnian Serb forces until late last year, when treacherous tracks were plowed across several mountains to link the city with roads leading south to Croatia. Those links opened the way for international relief organizations to begin working here.
Beslagic also has organized systems to distribute food and financial aid to the neediest 22,000 residents and provide 20 percent of the relief for some 60,000 refugees packing public buildings and private homes.
Taxes are collected, although evasion is extensive. There are limited telephone links to the outside, a regional railroad, radio and television stations, four newspapers, and some magazines.
The German mark is king, but United States dollars and other hard currencies are accepted.
Relative ethnic calm has been the key to the accomplishments here, cited by peace mediator Lord David Owen as proof that Muslims, Serbs, and Croats can coexist. Many Serbs and Croats, believers still in the pan-Yugoslav idea, fight in the Bosnian Army.
"I have a job and a place here," says Zoran Stakic, a Serb who enlisted. "I did not want to leave the life I made. I do not have any nationalist feelings. People like me have the biggest problem in all of former Yugoslavia. We have lost our country."
Mirza Kusljugic, a computer science professor and the director of Tuzla's self-sufficiency operations, attributes ethnic amity to a large blue-collar work force steeped in camaraderie and the brotherhood and unity ethic of the former communist Yugoslavia.
"There is also something mystical about Tuzla. We have a saying: `Salt and blood do not go together,' " he says, referring to the salt deposits that gave the city its name, a derivative of the Turkish word for the mineral. Signs of strain
But Tuzla's delicate ethnic comity, and all that it helped produce, is increasingly threatened.
Anti-Serb sentiments are rising, fueled by the brutal Bosnian Serb siege on the eastern Muslim town of Srebrenica, about 50 miles to the southeast.
More than 7,600 refugees have been evacuated by UN convoys in the past month from Srebrenica to Tuzla. A UN official who has spent considerable time in Tuzla says if Srebrenica falls, there could even be a violent backlash against the city's Serbs. "You can't control soldiers who are losing a war," the official warns.
The city exhausted its food reserves last October caring for its own people and earlier waves of refugees uprooted by Bosnian Serb "ethnic cleansing."
While the United Nations High Commissioner for Refugees (UNHCR) and other agencies are now providing 80 percent of the aid for refugees, local indignation is growing over the new influx.
Ethnic tensions also have grown out of a recent dispute with the regional government. The Party for Democratic Action - known by the initials SDA - is the nationalist party of Bosnia's Muslims. It offered Beslagic the regional chairmanship in what local officials saw as a ploy to gain control of Tuzla's resources.
Beslagic refused, prompting a vicious reaction from the SDA, which accused him of being anti-Muslim. The campaign failed to dent the mayor's popularity, but it did boost the insecurity of Tuzla's Serbs. "We feel disenfranchised by the regional government," a Serb woman says. Flow of goods limited
Tuzla is unable to obtain many of the spare parts and petroleum derivatives needed to keep its industries alive because of Serb road blockades and seizures of Tuzla-bound truck cargoes by Croatian authorities and Bosnian Croat militiamen.
Jose-Maria Mendiluce, the special UNHCR representative in former Yugoslavia, says Croatia and its Bosnian proxies also have restricted UN aid supplies.
"If we could solve the problem of transportation, we could very quickly solve the problems of our industry," Beslagic says.
The city has launched efforts to restock its food reserves, with residents digging vegetable gardens in parks and deserted lots. UNHCR has begun distributing seed potatoes. But Mr. Mendiluce says more international assistance is needed to preserve Tuzla's economic and social successes.
"We have to make an all-out effort here to revive the economy. We need to reorient our program from emergency to medium- and long-term, to invest more in the infrastructure," he says. "We have to help the people to help themselves."
That suits Beslagic fine.
"There is an old Chinese proverb," he says. "Don't give us fish. Just teach us how to catch fish."