I just finished watching
Shaun's feature-length video on the bombings of Hiroshima and Nagasaki. Needless to say, a video going over the details and causes of a historical tragedy makes me think of ways that that tragedy could have been avoided.
History stuff from that video:
Spoiler:
The Japanese Supreme War Council was composed of six members. At the end of the war, it was split between two three-man factions, the Moderates (including Admiral Mitsumasa Yonai, Minister of the Navy) and the Hardliners (including Admiral Soemu Toyoda, Chief of the Navy General Staff). To quote Shaun: "Generally, the Hardliners favored a more...hard-line military strategy and the Moderates were more...moderate."
By the end of the war, neither faction wanted an unconditional surrender, but they disagreed about the conditions they should request—the Moderates just wanted to make sure Emperor Hirohito wouldn't be harmed or removed from office, while the Hardliners had a few other things they wanted to push for. With three people in each faction, neither side could force the other to agree, which was convenient for the Hardliners since not making peace yet was their entire (short-term) goal.
Meanwhile, the USA has a couple of offices prominent in that video—the President and Secretary of State. (And others, but those are the ones I'm mentioning here.) To summarize and simplify, Harry Truman and his SoS Edward Stettinius Jr were more hardline on not offering concessions to foreign powers than FDR and SoS Cordell Hull were. Shaun also mentions that Truman, being informed of the Manhattan Project and plans to target Japan with nukes very late in the process, mostly stayed out of the way rather than interfering. If plans to force unconditional surrender were to be halted, it would probably need to happen before FDR's death.
Shaun identifies three things that needed to happen before the Japanese would surrender "unconditionally". The first (well, third in Shaun's ordering) was the aforementioned condition about the Emperor, which in our timeline the US gave them while still asserting that the Japanese were surrendering unconditionally using rhetoric that isn't important here. The second (first) was that the Japanese leadership recognize their military position as hopeless, which was true before '45.
The third (second) was that the leadership recognize their diplomatic position as hopeless. By the end of the war, the Supreme War Council had roughly two plans: Make any Allied invasion painful enough that they'd lose their will to fight (and hope that they didn't just continue blockades and bombing), and/or get the USSR to negotiate a peace between Japan and the Allies where they'd get better terms.
Shaun compares this diplomatic plan to how the USA had negotiated a peace between Japan and the Russian Empire a few decades prior, an example which is particularly important since it reminds us of why this negotiated peace was implausible. Stalin wanted to retake the land Russia had lost during the Russo-Japanese War, and secured that promise at Yalta—they'd get the land if they invaded Japan, two or three months after Nazi Germany surrendered (for logistical and opsec reasons).
Now, Japan didn't know about this agreement, but their ambassador to Russia did repeatedly point out that they weren't getting anywhere with that plan; Japan had nothing to offer that the USSR couldn't take. This had no effect, and the Supreme War Council continued to hope that the USSR would help them negotiate peace up until the USSR broke their non-aggression pact.
So here's my thought. Sometime early in 1945 (probably a bit before the Yalta Conference), Soemu Toyoda dies and the Moderates (with help from Mitsumasa Yonai) make sure a Moderate is chosen as his successor. Let's call this successor
Captain Juzo Okita.
Captain Juzo has two roles in this alt-history. One is to break the deadlock between Moderates and Hardliners; the other is to convince the Minister of Foreign Affairs Shigenori Tōgō (another Moderate) that Stalin won't bail them out. This leads to Japan actively seeking surrender around March, perhaps after the firebombing of Tokyo. FDR accepts their surrender in a way that makes it clear both that they're not getting rid of the emperor and that that doesn't count as a condition, letting them get everything they want out of the surrender.
At this point, Germany hasn't surrendered yet, but the war is basically over. The occupation and reconstruction of Japan start a few months sooner, and Japan is spared a few months of economic damage (and two nukes, of course). FDR still dies; the Trinity Test and the rest of the Manhattan Project continue, heedless of how they won't be used on their long-selected targets, and eventually a public demonstration is made to show the overwhelming power of the US military (a proposal made in our timeline before the generals decided to just bomb cities anyways).
Stalin is ****** that the other Allies made an agreement to let them take land from Japan and immediately invalidated it by ending the war before they could join, which doesn't help tensions between the USA and USSR at the start of the war. On the other hand, he didn't get a chance to break his non-aggression pact, so relations between the USSR and Japan might not be as bad, letting Japan potentially serve as a diplomatic intermediary between them and the USA? I dunno, I'm just trying to think of how this timeline could move forward.