On the matter of realpolitik and the idea that “nations don’t have friends, they have interests,” I’d like to start with some quotes from Thomas Jefferson:

The Moral duties which exist between individual and individual in a state of nature, accompany them into a state of society & the aggregate of the duties of all the individuals composing the society constitutes the duties of that society towards any other.

I am in all cases for a liberal conduct towards other nations, believing that the practice of the same friendly feelings and generous dispositions which attach individuals in private life will attach societies on the larger scale, which are composed of individuals.

I think with others that nations are to be governed according to their own interest, but I am convinced that it is their interest in the long run to be grateful, faithful to their engagements, even in the worst of circumstances, and honorable and generous always.

The interests of a nation, when well understood, will be found to coincide with their moral duties.

We are firmly convinced, and we act on that conviction, that with nations as with individuals, our interests soundly calculated will ever be found inseparable from our moral duties.

I think Jefferson understood that there was no way to prove assertions of the utility of morals and friendship in the dealings of nations.  Note in the quotes above that the predictive ones contain clauses like I think and we are firmly convinced, whereas the straight moral statements are presented without equivocation.

In this humility, I think, lies the essential difference between Jefferson and the realpolitik crowd.  The latter will have states behave abominably just to show how far they are above the naiveté of the common ruck.  In the end, though, it’s just vanity and showmanship, since they’re no more capable of predicting outcomes than anyone else.  They may in fact be smarter, and certainly may be more knowledgeable, but those differences evaporate before the complexity of causes that move world events.