Japan did have a nuclear program, but after Hiroshima were convinced that U235 was so hard to separate, their physicists reported it was a bad miracle, that it would take another year to make another bomb like that. The Nagasaki bomb showed reaction products from Plutonium which they knew was made from Plentiful U238, and any number could be made quickly. MAGIC decrypts gave insight into that level of strategic thinking, and also told us that they would claim their surrender was largely due to USSR entry, to try to deny US benefit of the bomb.
Fascinating. I’m thinking I must be the last person to know this.