There were people who have a lot of different viewpoints just as you have right now in our society. There was also a lot of discussion on war and peace as is now the case.

An interesting shift took place in the general discussion due to the coming of Christianity. War and the pacifist moral of Christianity don't mix to good. This led to thinkers finding answers to justify violence in warfare. Although this discussion about morality and warfare was already present, this impulse coming from Scholars like St. Augustinus led to the creation of a new school on warfare what we now call Just War Theory. In this tradition scholars try to make a list of requirements a war needs to have to make it Justified.

Of course a lot of other Scholars way before St. Augustine wrote about warfare like Cicero, Plato and Aristotle. Both Greek and Roman scholars shared and disagreed on the concepts of warfare. But their writings are still very relevant today and are cited much in contemporary debates.

This doesn't really answers any questions about warfare in practice since I don't know much about that subject. A very interesting read here is History of the Peloponnesian War by Thucydides. In this book Thucydides a Greek Historian and General gives an historical account of the war between Sparta and the Delian League