To add a specific answer to the longer answer linked to: 1. Japan did not offer unconditional surrender before August 15th (they offered conditional surrender on August 10th, which was rejected); 2. Prior to the use of the bombs, they did not offer surrender at all, but explored the possibility (in what they thought was secret, but the US was cracking their diplomatic cables) of getting the Soviet Union to mediate a conditional surrender agreement with the USA.

The main Japanese "condition" was preservation of the Emperor, which the US ultimately granted, but only after the Japanese agreed to accept unconditional surrender.

As for the motivation: the US policymakers had many motivations for using the weapons. Improving their postwar position vis-a-vis the USSR was indeed one of them for some of the policymakers, most explicitly stated by James Byrnes, the Secretary of State. But it was not the only motivation, and there isn't strong evidence it was ever a driving motivation.

Hi there! Not discouraging further answers, but this section of our FAQ and in particular this answer may be of interest to you.

Nice, thank you for your time.