Most of America’s wars during the past century have been justified on the basis that we were coming to the aid of oppressed peoples and liberating them, which, incidentally, is the same justification the Romans used for most of their imperial expansion.  The abuse of this justification has often led me to wonder whether such intervention is ever justified, and so I was intrigued to find the discussion of the question in Quaestio IV of the Vindiciae Contra Tyrannos.  Although he is actually about to argue in favor of foreign intervention on behalf of oppressed citizens, particularly Christians, Mornay begins by offering a very insightful caution: 

“And indeed there are many who have readily judged it to be lawful, once they have hoped to augment their own wealth by affording assistance.  For in this way the Romans, Alexander the Great, and many others frequently extended their frontiers on the pretext of repressing tyrants.  Not long ago we saw Henri II, king of France, waging war on Charles V under the pretence of delivering and defending the princes of the Empire, and Protestant ones at that; just as also Henry VIII, king of England, was ready to render assistance to the Protestants of Germany, in order to create trouble for Charles V.  But if there is danger to be feard, or little profit may be hoped for, then you will certainly hear most princes debating whether it is lawful or not.  And as the former concealed ambition or pursuit of gain under the cloak of piety, so the latter proclaimed the justice of their inactivity.”

It is this principle, of course, that explains why Kuwait was desperately oppressed enough to warrant a full-scale war, while Rwanda merited no intervention.