As a member of the Armed Forces, one has to be able to distinguish between “intentionally killing” non-combatants and the fact that some non-combatants may die as an unintended evil effect.
Nevetheless, it is never morally lict to do evil such that good may come from it.
The New Catholic Encyclopedia provides four conditions for the application of the principle of double effect:
*]The act itself must be morally good or at least indifferent.
*]The agent may not positively will the bad effect but may permit it. If he could attain the good effect without the bad effect he should do so. The bad effect is sometimes said to be indirectly voluntary.
*]The good effect must flow from the action at least as immediately (in the order of causality, though not necessarily in the order of time) as the bad effect. In other words the good effect must be produced directly by the action, not by the bad effect. Otherwise the agent would be using a bad means to a good end, which is never allowed.
*]The good effect must be sufficiently desirable to compensate for the allowing of the bad effect” (p. 1021).