Another theory is that the bomb was used because the USSR was shifting its focus towards Japan.<p>In Europe, all allied forced descended on Germany, and Europe the region was divided between the west and east, specifically the US&#x2F;Marshallplan&#x2F;NATO and the USSR&#x2F;Molotovplan&#x2F;Warschaupact.<p>Had the war ended in Japan the way it did in Europe, with the USSR playing a major role in the final battle and the inevitable Japanese peace process, Japan (and the region) may have been divided much like Europe was. Japan might not have become an ally of the US, might not have been part of the gigantic US economic motor in the post-war era that heavily relied on Europe and Japan as (economic) allies, and Japan may have ended up as some kind of North&#x2F;South Korean divide but between the US&#x2F;USSR instead of US&#x2F;China.<p>The theory goes then, that the nuclear bomb hastened the process to swiftly force a Japanese surrender before the USSR could exercise its influence in the region.<p>As such under this theory, the nuclear bomb is seen as the beginning of the cold war, which is normally recognised to have begun two years later. The bomb ensured the US its first major exclusive ally, Japan, in the cold war.<p>I haven&#x27;t studied it but it seems like a credible theory in that it&#x27;s plausible. Whether this plausible type of reasoning actually played a role in the bombings is another story, one I haven&#x27;t ever taken the time to explore.<p>In contrast, the notion that the USSR&#x27;s declaration of war made the Japanese surrender strikes me as implausible. After all, the Japanese had been viciously decimated for many months, the nuclear bombs were dwarfed by the firebombings campaigns of all major cities over and over again, and it was well known that the Japanese&#x27;s military capacity was reduced to virtually nothing, and that as a country with few natural resources and reliant on imports, it was bleeding dry after every single one of its logistics routes (air, land, sea) were completely cut off. Finally it had no air supremacy left and was defenceless to bombing. My point is, victory was impossible and defeat inevitably, the USSR declaration changed very little from the Japanese perspective. But it changed a lot for the American perspective, which is why it&#x27;s plausible the US wanted to force surrender quickly.<p>Beyond that, the cold war was very much, perhaps most powerfully characterised, by constant shows of power. Demonstrating nuclear power as the first nation must have played a role in the US&#x27;s claim to hegemony. Any doubts there may have been that the era of European supremacy was over, and that the US had become the world&#x27;s superpower, must have instantly faded. It&#x27;s the equivalent of China overtaking the US in GDP, building a base on mars, dominating at the olympics and showing unprecedented military power and weaponry. In that sense, too, a show of power with the bomb was perhaps the first action of the cold war.