In 1969, the two countries were in the middle of a 3-game match to qualify for the World Cup. Fights broke out and fans were injured. Honduras broke diplomatic relations with El Salvador and El Salvador invaded. The resulting "Soccer War" (or "Football War") lasted for 4 days and left thousands of civilians dead.
The war didn't just start because of a game. Tensions were high between the two countries. Honduras was experiencing economic problems and it was trying to expel Salvadoran immigrants back to El Salvador, which at the time was overpopulated. Honduras had also been enforcing a new land reform law by redistributing Honduran land from Salvadoran immigrants to Honduran citizens. In this context, all it took was a couple soccer games to cause a war to break out.