Nuke fails at Hirsosima
Let’s suppose that the Enola Gay dropped the nuke, but there was a loose wire or something wrong with the bomb and it failed to detonate. If it dropped all the way to the ground, it would probably smash to millions of pieces, but, to make things more interesting, let’s say it lands relatively intact.
Could the Japanese discover what it is? Could they use it themselves (and not by accidentally setting it off as they poke it, therefore scoring a massive own goal)? Would Truman drop the second bomb without knowing what had happened to the first? Any other possibilities?