This is a guest post from Simon Cotton, Australian National University, where he is a Visitor in Philosophy, and the University of New South Wales, Canberra, where he teaches in Humanities and Social Sciences.

Much of the commentary on Oona Hathaway and Scott Shapiro’s recent book, The Internationalists, including at Duck of Minerva, has focused on the empirical basis for their controversial thesis. Hathaway and Shapiro do not just claim that much of the decline in major interstate war that we have seen since the Second World War is down to mere reformulation of black-letter law, but that the 1928 Kellogg-Briand Pact of 1928, which appeared an embarrassment in its immediate aftermath, was pivotal to this transformation.

It is unsurprising, then, that political scientists have taken issue with their claim. In contrast, The Internationalists’ philosophical presuppositions have attracted less attention. This is a pity, because this work represents an invaluable opportunity to demonstrate the practical relevance of philosophy of law, an area that hard-headed social scientists are apt to dismiss.

Writing at Foreign Policy, prominent realist Stephen Walt argues that it is more likely that shifting incentives have been responsible for the decline in war. In particular, he claims that nuclear weapons have raised the security costs of war, trade and financial integration have raised its economic costs, and democracy has raised its electoral costs. As such, wars that would previously have passed a cost-benefit analysis have failed to do so at an increasingly rate since 1945.

On the face of it, Walt appears to favor shifting incentives at the expensive of law for methodological reasons. He says that if ‘changing norms’ were driving the decline in war we ‘should be able to point to numerous cases where national leaders had a clear incentive to expand their territory… and then decided not to go ahead… because they believed such an act was inherently wrong.’ Yet, Walt claims ‘as one reads the book, one searches in vain for direct evidence of this sort.’ Here, Walt is of course appealing to what Harry Eckstein termed the ‘crucial-case’ method. A crucial case is a one in which, if the outcome is observed (in this case, a war averted), then it cannot but be due to the factor hypothesized, as all other factors were absent.

There are problems with this criticism. First, why isn’t the burden on exponents of shifting incentives to point to crucial cases? How many cases are there of war being thought obligatory (consider an accountable government guarding the perceived entitlements of its citizens) and yet leaders not going ahead because of disincentives? Second, even were there no such case we could not conclude that law was impotent. Perhaps law remains sufficient to prevent war, it is just that law and incentives have tended, historically, to move together. Most importantly, however, law is most effective when it works by shifting incentives, as Hathaway and Shapiro point out in their response to Walt. As such, were they to offer the case Walt describes, they would vacate the factor behind their explanation rather than merely those behind competitor hypotheses.

It might thus be that Walt is really hung up on a problem that realists have thought insurmountable since Hobbes’s Leviathan. How could law shift incentives? Without a common government to enforce compliance, what good are mere words? In their reply to Walt, Hathaway and Shapiro seek to undercut this concern by offering a hypothetical. Specifically, they claim that it is doubtful the US would have invaded Mexico for nonpayment of debts in 1846 had it been illegal to do so. For one thing, resources, including those that come with territory, are less valuable to the extent that they are not in demand, and, they suggest, demand is dampened when resources are tainted by lack of legal ownership. What reason would the US have had to seize valueless resources?

Without elaboration, this response might be thought to beg the question. Why, wouldn’t the US have been so able to trade the resources it forcibly took from Mexico without a global government to punish purchasers for dealing in stolen goods? Of course, it is entirely possible that the latter would have been hesitant to purchase those resources if they had thought that they couldn’t sell them on. But this just pushes the question back a step. Where it is not already the case that countries are hesitant to buy resources seized in war because of a reasonable fear of not being able to sell them on, how could publicly designating such goods ‘stolen’ by reformulating the law change matters?

In his philosophical work Legality, Shapiro suggests an answer to this question by drawing an analogy between a set of laws and a plan. Just as having a plan can enable a group to achieve outcomes that would otherwise be infeasible—despite a plan being also, as it were, mere words—so too can law. (Indeed, Shapiro takes it that a legal system is a particular kind of instantiated plan.) For illustration, it’s entirely possible that at least some states in the 1840s would have been prepared to boycott resources seized through war provided that doing so would actually accomplish something. More relevantly, it’s entirely possible that, had only some states done so, other less-principled powers would have had a selfish reason to follow suit. Yet, without a legal prohibition to provide the role of a focal point, and around which states could coordinate, were any one state to boycott it would forgo benefits for, potentially, nothing. After all, unilateral boycotts accomplish little where markets are competitive.

None of this is meant to imply that the thesis Hathaway and Shapiro offer in The Internationalists is not vulnerable on empirical grounds. It may well be. But it should not be dismissed on the mistaken assumption that it posits something inconceivable, an assumption it is all too easy to fall into without an appreciation of the nature of law.