In August 1945 the Atomic Bombs dropped on Hiroshima and Nagasaki had some of us cheering. Also, there was a sense of awe that a single bomb carried by a single aircraft could do as much damage and worse than many nights of bombing by large air forces. It made us realise that the world had changed forever.
The Japanese War for the UK and the USA had begun in December 1941 with the declaration of war by Japan against Britain and the USA. This was a further stage in the bid to be masters of the East. This warfare had begun in October 1931 with attacks in Manchuria and later on China with all the attendant horrors.
They believed that victory would be theirs for the taking given that Britain at that time seemed to have little hope of stopping Germany and were vulnerable in the East. The problems in India helped to encourage Japanese confidence that it was just a question of pushing at the door.
As for the USA, they were perceived as being weak by Japan given the strength of isolationist opinion in American politics at the time and the never ending confusion of their electoral system. But what was critical was that the USA under Roosevelt had begun to rearm and Japan needed to hit hard before this was fully in place.
The rest of the story is well known enough. What I am certain of is that when the enormity of what the Japanese had done emerged as their troops stood down and the gates of the camps for internees and prisoners were opened there were very few people who regretted the dropping of the A Bombs.
There were some I knew, looking at the wreckage in many our own cities, who felt that the Allies should have gone on to A Bomb every urban area in Japan.