By Nick Carey CHICAGO - Driven to the brink of a crushing World Series defeat on Saturday, the Chicago Cubs pledged to mount a backs-to-the-wall rally in Game Five to keep their slim hopes alive. The 7-2 loss to the Cleveland Indians at Wrigley Field left the Cubs 3-1 behind in the best-of-seven Fall Classic, needing three successive wins to claim their first championship since 1908. The Cubs won three in a row to beat the Los Angeles Dodgers in the National League Championship Series, having come back from 2-1 down.