Sorry if this has already been asked but I keep seeing mainstream debate about whether or not if the dropping of atomic bombs was necessary, with the argument that the firebombing happening already put Japan on the brink of offering surrender. The topic always generates a lively debate and what is odd is that this never seems to be settled because, oddly enough too, I never saw any first hand sources given to explain the underlying motivations of the Japanese government to surrender which would either refute or support either side of the debate.
So Hirohito's surrender statement (the Jewel Voice Broadcast) does make a reference to the bombs:
But now the war has lasted for nearly four years. Despite the best that has been done by everyone – the gallant fighting of the military and naval forces, the diligence and assiduity of our servants of the state, and the devoted service of our one hundred million people – the war situation has developed not necessarily to Japan's advantage, while the general trends of the world have all turned against her interest.
Moreover, the enemy has begun to employ a new and most cruel bomb, the power of which to do damage is, indeed, incalculable, taking the toll of many innocent lives. Should we continue to fight, not only would it result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization.
I quote the two paragraphs and not just the end because it's important to see it in context. It's not just the bomb, but it serves as a useful capping event for trends already going on, and let's him claim that he's not only saving Japan by surrendering, but all of human civilization.
Which is pretty over-the-top. How much do we think that this broadcast reflects what Hirohito really thought, versus is something tailored to make the Japanese people accept it? More the latter than the former. We already know Hirohito wanted peace since the spring and summer of 1945. The atomic bombs, and the Soviet declaration of war, helped push the holdouts in the cabinet over the edge.
Beyond that: what we have in terms of sources are occasionally diary entries from the cabinet members, and more frequently after-the-fact recollections. The diary entries are not entirely clear though in some cases the effect of the bomb and the effect of the Soviet invasion are rather pronounced. The after-the-fact recollections vary and sometimes seem tailored to an audience, and are less reliable.
The result is a mixture of individual motivations; for some the bomb loomed large, for others the Soviets; for others it was more like Hirohito where they had already recognized the futility of continuing and were looking for an excuse. It's messy, as you'd expect something of this order to be when spread out across the personalities of the half dozen or so people who mattered in this decision.
Hasegawa's Racing the Enemy contains a lot of references to the sorts of sources that shed light on these things, and ultimately concludes that it was some mixture of the bomb and the Soviet invasion that pushed the cabinet over the edge of surrender, combined with a failed coup attempt that seems to have convinced the military that they had lost control of the narrative. It's a messy thing.