The principles of Just War are discussed in the Summa as well as the idea of Natural Law. One of the greatest gifts of Aquinas is a philosophical basis for an argument against relativism, making it as relevant to the modern world as it was the year it was written. Saint Thomas maintains that it is possible in almost every situation to draw bright lines between what is moral behavior and what is immoral behavior, between good and evil. It is also almost always that moral behavior is the hard choice. It has always been so.
Enter through the narrow gate; for the gate is wide and the road broad that leads to destruction, and those who enter through it are many.
How narrow the gate and constricted the road that leads to life. And those who find it are few.