Thomas considered war as well as the idea of war to be justified in certain cases and unjustifiable in others (Tooke, 1965, Pg. 77). Once these objections have been considered in-depth, we can evaluate how applicable they are to the world around us and if they still stand true today. As one of the greatest medieval thinkers, the works of St. Thomas Aquinas hold a special position when it comes to philosophy and ethics. On the subject of war, the Summa Theologiae is his most relevant work since he considers the idea of war itself to be a sin and thus all war is unjust as well as unlawful (Finch, 2007, II-IIq.40a.1). These objections stem from the Christian heritage which St. Thomas is quite attached to and other objections made to war also connect in some way or the other to the concepts given in the Bible.
For example, objections such as war being contrary to peace thus being a sin is certainly an idea that comes from Christianity but St. Thomas also uses logic to help him in his objections to war. For example, he considers the legality of war and suggests that a contest of reason would be perfectly legal since there are no deaths involved when reasons battle but when individuals engage in martial contests, they die without reason and thus die without legal causes.
However, St. Thomas does not dispute that a war can be justified in some situations but lays down strict criteria for a war to be just. He demands that war should be authorized by a prince who has the right to command the warriors to go to war. Individuals who do not have such authority cannot wage war and should not engage in the battle to settle their differences which can be settled in a court of law. A war can be justified if it is fought in self-defense and not as a war of aggression. Moreover, the war which is ordered by the prince can be fought against people within the land as with people from without the land in order to protect the kingdom.
St. Thomas Aquinas also demands that war should have a just cause, which is to say that war must only be carried out on those who have shown that they deserve a war to be imposed on them.