So I assume that you consider Hiroshima and Nagasaki to be war crimes, no?
Yes, but it is a subjective call.
From Wikipedia regarding collateral damage and war crimes:
Collateral damage is the unintended damage to civilians and non-military property during war, either accidental or as a side effect of actions that do not violate the laws of war. Although the concept goes back at least to Thomas Aquinas, some feel the phrase is used these days as a cynical euphemism for civilian casualties. The term came into popular use among the general public during the 1991 Gulf War.
Regarding the deliberate targeting of civilian populations, by military or non-military personnel, see terrorism.
In war, there are in principle legitimate military targets and illegitimate civilian targets. If civilians or their property are affected, it is nevertheless not a war crime if a) that effect could not be foreseen or b) it could not be avoided and was minor in relation to the military objective. Clearly, because whether damage is collateral or not depends on judging internal states and what counts as minor, it can be difficult to judge in individual cases whether civilian loss was a war crime or collateral damage.
The intentional bombardment of Charleston civilians was clearly avoidable and thus a war crime by the definition above. The Feds aimed at the steeple of St. Michaels Church in a civilian area of town.
The Federal commander demanded in an unsigned note sent in the middle of the night that the Confederates surrender Morris Island and Fort Sumter or he would start bombarding civilians, which he did that same night at 1:30 AM. My interpretation is that he wanted to bomb Confederate civilians and set up the circumstances so that that would happen. This senseless and vindictive bombardment continued off and on for 18 months, long after Morris Island was in Federal hands.