You're right, America didn't intentionally engineer the death of millions until after the war, when it became the undisputed global superpower.

...If you choose to intentionally ignore slavery in the American South, Convict Leasing, and Jim Crow.

see also what the CIA got up to in Indonesia: https://en.wikipedia.org/wiki/Indonesian_mass_killings_of_19...

And in your living memory: Vietnam, Korea, the vile shit we did to Haiti and Chile, the 'Banana Republics', Iraq, Iran, Afghanistan, unconditionally supporting Israel and the Gulf States various war crimes and genocides, etc...

And of course, most recently, the sudden and complete excision of life-saving aid to children in Africa, leaving the world's most vulnerable to starve.

And also the total embargo on Cuba, to ensure starvation, with the intention of conquest, resource extraction, and collective punishment.

but God forbid someone on a message board tarnish the US's sterling reputation (relative to some of history's worst) during WWII.