In the dunes amid trees and brush next to Baker Beach in San Francisco is what looks like an old, rusted pipe sticking mysteriously out of the sandy ground. Beachgoers, picnickers and children clambering in the bluffs around the beach have ambled by the steel pole for more than six decades, unaware that it represents one of the scariest moments in San Francisco history. The pole was used to mount a 30-caliber, water-cooled machine gun, while the caved-in trench next to it provided cover for a crew of four soldiers ready to strafe the beach and waters at the first...