An article by Tony Coady in The Age usefully highlights those just war principles in light of which the bombing of Hiroshima and Nagasaki were war crimes. One key point, of course, is the combatants / non-combatants distinction:
There are those who deny the significance of the distinction between combatants and non-combatants. They think modern war is "total war", whole nations or peoples against each other. There can be no philosophical justification for this, and it has the consequence that there can be no moral distinction between killing an invading enemy soldier out to kill you and bayoneting the children in an enemy kindergarten or intentionally bombing a civilian hospital. This position might suit Osama bin Laden, but it should be anathema to those who oppose him.As that concluding sentence suggests, something else which can be drawn from Coady's argument is the following: if we allow that political leaders may choose to disregard the prohibition on deliberately killing civilians where they foresee a beneficial outcome from doing so - in terms, for example, of lower all-round casualties or other costs - then this becomes an option not only for Harry S Truman but also for the leadership of terrorist organizations. The laws of war will stand, as a result, merely as one consideration among others, and may be set aside in favour of a utilitarian calculation of what will work best overall. Those laws would cease to have any force.