Aquinas offers an excellent sketch of the dynamic of nature and grace in civil government in On Kingship, recognizing that, while a civil government will never be able to guide society to true beatitude and virtue in itself, since only the divine power in the Church can accomplish that task, it must nevertheless orient society in that direction, rather than pretending to provide merely neutral human goods. Human goods cannot be neutral, because true human good is found in God; therefore the government must manage the resources of society so as to pursue that true good, while humbly recognizing that the full accomplishment of this task lies outside their power. But don't listen to my poor attempts at summary--he says it much better:
"As the life by which men live well here on earth is ordained, as to its end, to that blessed life which we hope for in heaven, so too whatever particular goods are procured by man's agency--whether wealth, profits, health, eloquence, or learning--are ordained to the good life of the multitude. If, then, as we have said, the person who is charge with the care of our ultimate end ought to be over those who have charge of things ordained to that end, and to direct them by his rule, it clearly follows that, just as the king ought to be subject to the divine government administered by the office of priesthood, so he ought to preside over all human offices, and regulate them by the rule of his government.
Now anyone on whom it devolves to do something which is ordained to another thing as to its end is bound to see that his work is suitable to that end; thus, for example, the armourer so fashions the sword that it is suitable for fighting, and the builder should so lay out the house that it is suitable for habitation. Therefore, since the beatitude of heaven is the end of that virtuous life which we live at present, it pertains to the king's office to promote the good life of the multitude in such a way as to make it suitable for the attainment of heavenly happiness; that is to say, he should command those things which lead to the happiness of Heaven and, as far as possible, forbid the contrary."