In what circumstances will a federal court dismiss an action for failure to state a claim pursuant to Rule 12(b)(6)?

We review de novo a district court's dismissal of an action for failure to state a claim pursuant to Rule 12(b)(6). Tanner v. Heise, 879 F.2d 572, 576 (9th Cir.1989). All allegations of material fact are taken as true and construed in the light most favorable to the plaintiff. Id. In civil rights cases, where the petitioner is pro se, the court has an obligation to construe the pleadings liberally and to afford the petitioner any benefit of the doubt. Bretz v. Kelman, 773 F.2d 1026, 1027 n. 1 (9th Cir.1985).