Paris — French President Jacques Chirac, intervening after six nights of rioting in suburban Paris, called Wednesday for calm and said authorities will use a firm hand to curtail what may become a “dangerous situation.” The violence, sparked initially by the deaths of two teenagers, has exposed the despair, anger and deep-rooted criminality in the poor suburbs, where police hesitate to venture and which have proved fertile terrain for Islamic extremists. “The law must be applied firmly and in a spirit of dialogue and respect,” Mr. Chirac said at a cabinet meeting. “The absence of dialogue and an escalation of...