France were not underdogs in 98, they were favorites. They had reached the semifinal of Euro 96 with a squad of relatively young players like Zidane, Desailly, Blanc, Deschamps, Barthez, Thuram, Djorkaeff, Karembeu, etc. By the 98 World Cup those were established players at Europe's biggest clubs. 3 of their players were in the Champions League Final that year. Look at the squad:
http://en.wikipedia.org/wiki/1998_FIFA_World_Cup_squads#France
Bookmakers' odds for 1998 World Cup (The Times):
Brazil 3/1
France 11/2