SOUTH LAKE TAHOE, Calif. — They sent thousands of firefighters, 25 helicopters and an arsenal of more than 400 fire engines and 70 water trucks. Yet the fire still advanced. They dropped retardant chemicals through an ash-filled sky and bulldozed trees and brush to slow the march of the flames through the steep and rugged terrain of the Sierra Nevada. Yet the fire still advanced. Bursting across a granite ridge into the Lake Tahoe basin, the Caldor fire now threatens tens of thousands of homes and hotels that ring the lake. On Tuesday, the smoke-choked streets of South Lake Tahoe,...