Atomic bombing just?

Ernest W. Lefever reviews A. C. Grayling's Among the Dead Cities: The History and Moral Legacy of WWII Bombing of Civilians in Germany and Japan. Noting that traditional just war principles protect civilians from deliberate attack, Lefever asks whether the carpet-bombing of German and Japanese cities in the closing months of the Second World War violated these principles. Grayling says yes, and for the most part Lefever agrees. But he disagrees on the atomic bombing of Hiroshima:

Hiroshima was a tragedy, but it was also a necessary and prudent act in an eminently just cause. We Americans can regret the wrenching necessity for the atomic bombing, but we should not feel guilty about it.

Since Lefever stops short of saying the attack was just, is he then implicitly arguing that it may be prudent to commit unjust acts in the interest of pursuing a just cause? Can it ever be right to target defenceless women and children in order to conclude a war effort? And if so, how does this differ from contemporary islamist terrorists who see the 9/11 attacks as legitimate warfare against the dar al-Harb?