Call it attacks on civilians.
As much as the London blitz was horrific, it can't be considered a terrorist attack. Both countries were engaged in a declared war.
If that's the standard, the attacks on Hiroshima and Nagisaki killed about a million (eventually).
Civilians can't be specifically targeted. However major cities are fair game, since they house the leadership and strategic commands - and possibly military targets (both offense and defense). If civilians were the target both the United States and Germany would have been brought to trial for war crimes. Neither was, but Germany was brought to trial for war crimes based on the holocaust, which specifically targeted civilians.
Not so simple. under IHL (international humanitarian law, also known as the law of war or the law of armed conflict) military kinetic action must follow the principles of distinction, necessity and proportionality. This essentially means that the action must be a. directed at a military target (command and control centers may qualify..i.e. Hitler even though he didn't wear a uniform most of the time); b. necessity (there must be a military purpose to the attack) and c. proportionality. This means that the effects of the attack (including collateral damage) must be proportional to the value of the military objective. in other words, you don't blow up a city block full of civilians to kill one foot-soldier. such action may, however, be legitimate if the purpose was to, say, take out the control center for the entire enemy war effort.
put differently, taking a kinetic action knowing that civilians will inevitably die , does not in itself constitute a war crime. belligerents are permitted to carry out proportionate attacks against military targets even when it is known that some civilian deaths or injuries will occur. the crime is when the predicted incidental civilian injuries would be clearly excessive in relation to the anticipated military advantage -- violation of proportionality
WWII is a difficult example because you had a total world war with existential consequences (see the eastern front especially). some Nazi commanders were indeed tried and convicted for indiscriminate attacks on civilian targets (and the allies did prosecute some of their own for war crimes)....the area bombings of cities is still controversial....but consider that the German munitions infrastructure was largely located in urban areas (at least initially) and that the munitions of the time were not very precise (the Noren bombsight notwithstanding)...making area effect bombing necessary.
eta: speaking only for myself and not for the Department of Defense. however, I will note that the foregoing is simply the standard academic explanation of IHL.