In wartime there will definitely be accidental deaths, as there are accidental deaths on the highway every day. We cannot have a modern transportation system without expecting accidental deaths; we can even determine how many we expect to occur. The concept 'accidental' seems very clear to everyone when we speak about the highways, though, and becomes muddled when it comes to war.
I think there is a perverse alliance between hawks and doves when it comes to distinguishing between accidental deaths and on-purpose deaths. Both hawks and doves would prefer to keep the distinction unclear, for different reasons. Doves want to be able to call all deaths in wartime murder; conversely, hawks want to be able to take what is really deliberate murder and categorize it as morally licit collateral damage.
The moral reality is more difficult than either would like to concede. If I use a modern weapon to indiscriminately blow up a whole group of people, some of whom I know are innocent and some of whom are terrorists, the deaths of the innocents are not an accident. I killed them on purpose, precisely because they got in the way, and the technical capabilities of my weapon did not allow me to kill the terrorists without killing the civilians. On the other hand it is not merely permissible but noble and valiant for me to risk death and kill the terrorist himself, to defend the innocent from him. If I drop a smart bomb on his safe house, to the best of my knowledge clear of innocents, that is, clear of individuals who are not engaged in attacking behaviors, then I have done a good and noble deed. If my smart bomb misfires and kills some civilians, it is indeed an accident: the attack did not go according to my plans, and innocents were killed. There is such a thing as morally licit collateral damage.
If my plans for this specific attack entail killing some specific identifiable civilians, even if I wish I could make different plans which did not entail killing those particular civilians, then what I am planning is murder.
So the moral situation is I think tactically far more difficult and perilous than 'realists' would like it to be. 'Realists' would like to move forward unconstrained by the fairly clear moral difference between accidental and deliberate killing, because doing so would in fact make for a more effective war with fewer casualties. These 'realists' want to blame our deliberate murder of civilians on the other guy, because the other guy created and forced the circumstances which made us view it as necessary to murder civilians. Circumstances which make murder a very tempting option do not excuse it though; and it is better for a thousand to die by accident than it is to commit one murder.
Pacifists on the other hand want to be able to condemn violence tout court: to the pacifist every act of killing is murder, and especially even genuinely accidental civilian deaths in wartime are murder.
This results in a perverse obscurantist alliance between the hawks and the doves; which is to say, the obscurity of the term 'accidental' in wartime, compared to in peacetime activities like driving on the freeway, is no accident.