TIL the US military had plans for the invasion of mainland Japan named Operation Downfall. It was estimated that there would be 500k-4m US casualties and 5-10m Japanese casualties.

https://www.history.navy.mil/about-us/leadership/director/directors-corner/h-grams/h-gram-057/h-057-1.html

https://www.reddit.com/r/todayilearned/comments/165vaim/til_the_us_military_had_plans_for_the_invasion_of/