Historians' opinions about this have changed over time -- the generation of people who experienced it felt that a land invasion of Japan would be ruinous, and w

Historians' opinions about this have changed over time -- the generation of people who experienced it felt that a land invasion of Japan would be ruinous, and would kill a million US men. To that effect, the US tried firebombing cities, paraded battleships along the coast, sent subs to the Inland Sea, and telegraphed that they would destroy a city completely. Then they did. Meanwhile, the USSR invaded and militarily routed the Japanese. For the Japanese, the situation was militarily hopeless and had been for a while, but the events of August made this very clear to a wider audience. Perhaps there was no single event that triggered the surrender, but opinion, even of ministers, can cascade rapidly. Once one person publicly lost faith, it became easier for others to join him, but that might not have been initiated by a specific event or on a specific timeline.