Given that the Japanese resistance got exponentially more rabid as we got closer and closer to the homeland, there is no rational way to believe they would ever surrender without a nuclear attack.
Remembering that they still had more than 5,000 effective military aircraft and enough fuel reserves to send them out on one-way bombing missions. As well as fast, light coastal vessels that could accomplish much the same thing in the event of any D-day crush of landing craft, especially at night. And at least enough hand grenades and mortar rounds that every *civilian* man, woman and child over the age of five years would have one.