It turns out that Proudhon's answer to the musical question is rather interesting, and challenging. His two-volume War and Peace represents an further exploration of some of the ideas he had developed in Justice in the Revolution and in the Church. The turning point in Proudhon's philosophy came in the 1850s, between the Philosophy of Progress and Justice, when he realized that, as he later put it, "the antinomy does not resolve itself." The immediate consequence of this realization was a move from the emphasis on synthesis, which had dominated his work from the last sections of What is Property? through the System of Economic Contradictions and beyond, to a much more complex treatment of the role of ultimately irresolvable conflict in social evolution. One consequence of this change in his dialectical approach was that the historical accounts that made up such an important part of Proudhon's work had to be revisited, in order to determine if and how this other dynamic revealed itself. Another, linked, consequence, was that the philosophy of progress and the theory of collective force and collective beings had to take what we might now consider an "anti-foundationalist" turn.
For the later Proudhon, the series (taking this term with all its Fourierist implications) of approximations of Justice, the successive balancings of more-or-less free forces, was Progress (big-P), even the Revolution, and there was something relentless about its upward climb. The theory of collective persons, what contemporaries like Pierre Leroux and William B. Greene addressed (with some variations) as a "doctrine of life" and theory of Humanity, made possible the theory of "immanent justice," which posited at least the constant possibility of advance, in the form of improved approximation. But that same theory meant that there were lots of actors on the social-historical stage, lots of kinds of actors, to which individual persons had a variety of kinds of connections and in which they had various investments. Add to that the additional wrinkle that each "stage," each element in the historical series, lingers. Nothing is magically "realized and suppressed" by synthesis, and each approximation ends up resting on other approximations -- all the way down, really.
For Proudhon, Justice, despite its key-word status, is never in-itself anything more than that balance of free forces. "An eye for an eye" is justice, as is The Golden Rule: they simply occupy difference places in the Justice-series. Obviously, "higher" approximations of justice have their advantages, both for individuals and for collective beings like Humanity or a given society. But the existence of "higher" approximations does not necessarily invalidate the "lower," particularly if, in the historical series, later, higher approximations are founded on those earlier and lower.
Proudhon's War and Peace is one of those texts routinely cited as a forerunner to "fascism" (a term that requires scare-quoting, not because there is any question that fascism has existed, but because the critics tend to lump a lot of "stuff we don't approve of" into the mix.) The literature on proto-fascism is a complex one, frequently involving the defense of certain models of rationality and science, as well as the particularly political and ideological forms we associate with the term. So Bergson can be blackened with Sorel's various political indiscretions, presumably because his treatment of "intuition" is "anti-science" and "anti-rational," rather than part of a debate about what science and rationality will be. Proudhon, already an undesirable for that "property is theft" stuff, gets the "insufficient degree of separation from Sorel" treatment, and his anti-semitic notebook entries are mentioned, and who would dare argue that his War and Peace was not an irrationalist glorification of war, even if its final line is "HUMANITY WANTS NO MORE WAR." Hey, he was "a man of paradox." Right?
No. But thanks, I guess, for playing...
Proudhon does, in fact, talk about war as having an important moral function. He talks about the extent to which it has been war which has driven human beings to acts of bravery, self-sacrifice and ingenuity. If he doesn't quite get to Marinetti's "war is the world's only hygiene," he does point out that we have relied pretty heavily on war to maintain what balance of forces we have achieved. It may not be nice to say so, but it doesn't appear to be incorrect. And the critics of war don't seem to deny the basic right of force, when push comes to shove, or to class war, or General Strike, etc. What Proudhon attempts to do, in a work which is not always a comfortable read (as if we required comfort from political philosophy or history), is to demonstrate the ways in which the right of force (not a right to force, about which more a little later) has functioned in the service of Justice, has contributed to the subsequent approximations of Justice, and continues to play a narrowly delimited role in the defense of Justice.
If you want to get a taste of how Proudhon argues in War and Peace, I've translated 17 pages from the first volume, where Proudhon is explaining the "right of force." Here are Chapter XI and the Conclusion of Book Two of Volume One.
A couple of things to remember, or to consider if you haven't read my other discussions of Proudhon:
1) He uses the French word droit, which can mean either "law" or "right" in a way that is most accurately translated, as far as I can see, as "right," but which does not, or does not necessarily refer to the sort of natural or political rights we are accustomed to talking about. Every group, ensemble, being, etc., has its own "law" (loi) of organization, which determines what is "right" (appropriate, proper, logical, natural, etc) for it to do. Likewise, it participates in larger ensembles with their own laws, which condition those of the individuals. Droit remains something of a mix of what we might call natural law and/or right, as well as covering more strictly descriptive (rather than normative) grand. Just don't necessarily assume that Proudhon is trying to anything more than describe the normal functioning of presently-existing "bodies" of one sort or another.
2) This is part of an explicitly historical, progressive account. The basic argument of the book is that all of our "higher-level" rights, and really all of our more peaceful institutions, as well as all those which we have yet to create, are part of a historical series which begins with relations mediated by raw force. Peace would be the end of that series, presumably, but war would always be its origin. Peace is, in a strong sense, the end product of the process of war. And, Proudhon says, we have got ourselves into some real trouble by denying this historical fact.
3) Proudhon speaks of "right of force" and "right of war," but these, he argues, are like all true rights, equal and reciprocal. If there are, or have been, certain circumstances in which the right of the strongest has been or can be our model for justice, justice is still a restless demand for balance, and ultimately justice cannot be fulfilled at any acceptable level by silencing or excluding the weak. It is not clear that there is a "right to war," but instead a sort of protocol for dealing with the wars that occur, and which can only justly occur in circumstances of social or political imbalance or injustice. Proudhon talks about the "right of labor" to its product, and contrasts that with the "right to work."
The translation is very rough and literal. I wanted to get through enough of this to clarify for myself how it fit with the things I'm working on more seriously. I'll try to clean it up sometime, but probably not for a few months.
Note: Parts of this post were lifted directly from a discussion at the Forums of the Libertarian Left which includes more discussion and context.