How did Truman's decision to use the atomic bomb forever change the nature of international relation?
From the time that the US used atomic weapons against Japan at the end of WWII, the nature of international relations was changed. This was because all of a sudden a war between major powers had become much more horrible of a prospect than it ever had been before.
Before the advent of atomic weapons, war was simply another part of international relations. Even major powers were relatively willing to go to war with one another. After atomic weapons were invented and used, it was no longer anywhere near as feasible to have a major war. Everyone knew that a major war would become a nuclear war, with disastrous consequences. This changed international relations fundamentally because it removed war as a serious possibility among major powers.