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For one thing, we can presume that we should fight with the right intention, that is, we must intend to punish not just anyone, but only evildoers. Likewise, we should do our best to see that our use of force does not detract from our duty to uphold the good. Of course, the ability to target only those who deserve to be punished, no less than the capacity to formulate plans of action that will issue in more good than evil, must be cultivated. Thus, for Aquinas, right conduct in war is dependent upon the virtues of soldiers and the commanders who lead them.

Unlike Aquinas, Calvin has little to say about the requirements for a just war, but he does insist that the lawful sovereign has a duty to take up arms to defend the commonwealth against those who attack it (Institutes IV.20). Calvin also insists that wars should not be waged in anger, nor in order to vent passions on others. It is a sign that we have fallen short of beatitude when our passions lead us to use force unjustly. This is why Calvin argues that princes should go to war only when driven to it by necessity and out of concern for the public good. Lastly, because soldiering is conceived as an office of love, Calvin rejects outright mercenary soldiering (a popular profession among the Swiss of Calvin’s time), since it encourages soldiers to fight merely out of love of money and not out of love for their neighbors. For Calvin, soldiering loses its Christian function and legitimacy when it becomes a commodity.

... In practice this means, among other things, that acts of force must never involve intrinsic evil (such as intentionally killing innocent people, for instance).

...a failure to engage in a just war is a failure of virtue, a failure to act well. An odd corollary of this conclusion is that it is a greater evil for Christians to fail to wage a just war than it is for unbelievers.

First-rate.