I would skip the bit about 'what makes a just war' as anyone fighting feels justified (wether rightly or wrongly), and in most cases are justified. Even from opposing points of view.
I would engage the topic 'How does one fight a war?' assuming that the war is justified. The point being that even when on the moral high ground, if you terrorize (torture, humiliate) the enemy, you lose the moral high ground, and place them on the moral high ground.
For eg, during world war 2, Italian pows held in Pietermaritzburg, South Africa, were given money and allowed to go into town unattended to spend it!
See this link for more details.
(Sleepy Hollow - LS Ongley - Biography
Thus, the stories the Italian pows sent home, placed the allies on the moral high ground, and the Italians were all too ready to turn against the axis. By contrast, A pow in the axis camp, was shot dead for merely retrieving a football that was out of bounds.
Winning the hearts and minds, is the antithesis of torture. If you can convince the enemy that it is better to be a pow in your camp, than a soldier in his army, you have already won the war. (bar the shooting)
Think Guantanamo bay, and how it has made the whole world turn against America, despite the initial invasion of Iraq (by Bush I in 1991 after Kuwait) being completely justified.