We also had convinced Japan that we had 100 more atomic bombs. There would be no way to defend against invasion with atomic bombs being dropped on their defending force.
We actually were going to use a bunch of A-bombs to clear the way ahead of our invasion.
Note this discussion between General John E. Hull (one of General Marshall's top aides) and Colonel Lyle E. Seeman (one of General Groves' top aides) on August 13, 1945:
General Hull: That is the information I wanted. The problem now is whether or not, assuming the Japanese do not capitulate, continue on dropping them every time one is made and shipped out there or whether to hold them up as far as the dropping is concerned and then pour them all on in a reasonably short time. Not all in one day, but over a short period. And that also takes into consideration the target that we are after. In other words should we not concentrate on targets that will be of the greatest assistance to an invasion rather than industry, morale, psychology, etc.
Colonel Seeman: Nearer the tactical use rather than other use.
General Hull: That is what it amounts to. What is your own personal reaction to that?
Colonel Seeman: I have studied that a good deal. Our own troops would have to be about six miles away I am not sure that the Air Forces could place it within 500 feet of the point we want. Of course, it is not that "pinpoint". Then the stage of development has to be considered. The work it is liable to be used for the more or less has to be explosive effect. It would be just a gamble putting or sending those troops though.
General Hull: Not the same day or anything like that. We might do it a couple or three days before. You plan to land on a certain beach. Behind which you know there is a good road communication and maybe a division or two of Japanese troops. Neutralization of that at some time from H Hour of the landing back earlier, maybe a day or two or three. I don't anticipate that you would be dropping it as we do other type bombs that are in support of the infantry. I am thinking about neutralizing a division or a communication center or something so that it would facilitate the movement ashore of troops.
Colonel Seeman: That is the preferable use at this time from that standpoint. The weapon we have is not a penetration weapon. The workmanship is not as good as possible. It is much better than average workmanship. We are still developing it though.
General Hull: From this on more or less of the timing factor, how much time before the troops actually go into that area do you think would be the safety factor? Suppose you did get a dud or an incomplete explosion, what safety factor should you consider, one, two, three days?
Colonel Seeman: I think we are sending some people over to actually measure that factor. I think certainly by within 48 hours that could be done. Everything is going so fast. We would like to train people and get them in a combat spirit to do that. I think the people we have are the best qualified in that line. Of course, as you say, if it is used back in a kind of reserve line or in a reserve position or a concentration area but that you wouldn't be up against right away.
General Hull: I don't think you would land at eight o'clock in the morning and you would drop it at six o'clock, out the day before, even from the tactical standpoint without regard to when it fails to go off or something like that.
Colonel Seeman: Another thing you may be likely to consider is that while you are landing you might not want to use it as it could be a dud. It is not something that you fool around with.
https://nsarchive2.gwu.edu/NSAEBB/NSAEBB162/72.pdf