Stalin’s 20 million and Mao’s 70 million both come from a book called ‘The Black Book of Communism’ where th authors tried to inflate all the numbers as much as possible to arrive at 100 million deaths under communism in the end. The really high numbers come from taking the difference between the population decades later and what the population would have been if the borth rate hadn’t dropped (even though lower birth rates are an indication of better living standards).
Also, if you leave out the USSR’s purges (Soviet archives revealed around 800k), almost all of the deaths come from ‘man made famines’. Completely disregarding that in both China and Russia there have been famines all the time before their revolutions. Both countries were among the poorest and most backward in the world. No government on earth could have prevented all famines immediately. After some years after Mao’s and Stalin’s reorganizations of agriculture, famines were eliminated (except for during WW2). Life expectancy doubled within 30 years under communist China. Very quickly in the Soviet Union as well.
Of course you can argue about the governments having been able to do it better in hindsight, but even then the deaths certainly weren’t cold blooded murder.
Considering this, you certainly won’t arrive at the tens of millions of people killed just by the US military. Many, many more if you factor in indirect deaths (by embargoes, sanctions, refusing to allow other countries to produce their vaccines, etc.).
Impossible with modern reactors, technologyimproved a lot since then.