It must be necessary, i.e., the last resort for restoring justice. The good to be attained must be greater than the attending evils. Only so much force may be used as is necessary to achieve the ends. There must be a reasonable prospect of success. Yet it is an attitude that gets a cool reception from two completely different schools of thought.
Some people see no place for moral considerations in matters of national interest.
They are not impressed by appeals to morality as such. For them actions must be justified on grounds of expediency. On the other hand there are many people, moved only by the highest moral considerations, who reject the idea that any war can be just. Further, they find any effort to maintain any kind of rules or international law in the conduct of war as being completely incongruous. In between there are those who point out that some rules about the conduct of war have been observed to some degree over the centuries.