The soccer World Cup is coming, which means a flurry of desperate attempts by tournament promoters to excite Americans about an event that electrifies the rest of the world. This year is no different. ESPN, which will broadcast most of the games in the United States, is airing a series of ads with members of the rock band U2. In one, Bono says the World Cup "closes the schools, closes the shops, closes a city and stops a war." If stopping a war seems like an exaggeration, another ad explains soccer's peace-building qualities in more detail: "After three years of civil war, feuding factions talked for the first time in years, and the president called a truce. Because the Ivory Coast qualified for the World Cup for the first time. Because, as everyone knows, a country united makes for better cheerleaders than a country divided." Does the World Cup really put a stop to war? Does soccer, known for its dangerously rowdy fans, have the conflict-reducing powers that ESPN and U2 proclaim? To be charitable to the World Cup, which this year will be held in Germany starting June 9, the evidence is mixed. It is undeniable that soccer has the power to unite - but its power to divide should not be underestimated. The belief that sports can be a source of peace dates to the start of the modern Olympic movement. But social scientists are split on whether competitive sports reduce or inflame conflict. A 1973 article by Richard Sipes in the journal American Anthropologist distilled the debate into two simple but contrasting arguments. One is that combative sports and war are substitutes for aggressive behavior - that the presence of sports is a healthy way for people to discharge their competitive urges. The other is that sports induce a warlike attitude, abetting conflict rather than reducing it. Sipes tentatively concluded that sports foster aggression. It is possible, however, that the worldwide appeal of soccer (well, minus the United States and Canada) has a pacifying effect. Former German foreign minister Joschka Fischer recently declared that once the tournament starts, "a football will become the symbol of our One World." There are certainly tangible examples of soccer soothing the savage beast of war. What did the British and Germans do during the famous 1914 Christmas truce across the trenches during World War I? They played a soccer match (the Germans won, 3-2). During the peak of popularity for Brazilian soccer phenom Pele, the combatants in the Biafran war in Nigeria declared a two-day truce so they could watch him play. Of course, in both cases, the cessation of conflict was only temporary. Soccer has also functioned as a useful outlet for postwar grievances. For generations after World War II, the conflict resonated in soccer matches between the Netherlands and Germany. Franklin Foer, editor of The New Republic and author of "How Soccer Explains the World," argues that the Dutch did not fully recover from the war until Dutchman Frank Rijkaard spit on Rudi Voller's mullet during a 1990 second-round World Cup match. Rijkaard's loogie was the only shot fired in restoring Dutch pride. Successful teams have also provided the occasional boost for national comity. The Ivory Coast example cited in the ESPN ad works here. Since 1999, the country has been mired in coups, rebellions and ethnic conflicts. When the national team - the Elephants - qualified in October 2005, the head of the Ivory Coast Football Federation pleaded with President Laurent Gbagbo to restart peace talks. Elections are scheduled for October of this year. While a truce is in place, however, Human Rights Watch warned in May that both government and rebel forces were devoting their energies to terrorizing civilians. THE PROBLEM is that historically, soccer has been just as likely to be the trigger for war as the trigger for peace. The best-known example took place in June 1969 between El Salvador and Honduras. Immigration and border disputes between the two countries had reached a boiling point at the same time that a three-game elimination match between the two national teams was taking place. Rioting during the second game led the two countries to break diplomatic relations. Two weeks later, the 100-hour Soccer War took place, resulting in about 2,000 casualties. Soccer also played a role in the run-up to the Balkan wars of the 1990s. In March 1990, Red Star Belgrade, a Serbian team, faced Dinamo Zagreb, a Croatian team, in the Croatian capital for a league title, a scant two weeks after Croatia elected nationalist Franjo Tudjman as president. According to Foer, that day was the first time in a half-century that Serbs and Croats openly fought each other. Red Star and Dinamo fans became so violent that the Serbian team had to be taken away by helicopter. Fifteen years after the match, the Zagreb daily Vecernji list observed, "The game that was never played will be remembered, at least by the soccer fans, as the beginning of the Patriotic War, and almost all of the contemporaries will declare it the key in understanding the Croatian cause." The leader of Red Star's ultra-nationalist fans - the Delije - was the notorious Arkan. He later recruited from the Delije to form the paramilitary force that engaged in ethnic cleansing of Croats and Muslims during the war, and ultimately was the victim of a gangland-style killing. While success at the World Cup can bolster national pride, losing can reap the whirlwind. A working paper by business professors Alex Edmans, Diego Garcia and Oyvind Norli finds that "losses in soccer matches have an economically and statistically significant negative effect on the losing country's stock market." Some individual players suffer consequences worse than that. Colombian defender Andres Escobar, responsible for an own goal in a 1994 World Cup loss to the United States, was killed upon returning to his hometown of Medellin. Soccer will never bring about peace on its own. The flip side is also true - by itself, soccer cannot start a war. The World Cup, like the Olympics, suffers from a case of overblown rhetoric. Bono's assurances to the contrary and the passions inspired by the World Cup embody both the best and worst forms of nationalism. Daniel W. Drezner will be an associate professor of international politics at the Fletcher School at Tufts University starting in the fall. Author e-mail: firstname.lastname@example.org .