(This piece is the latest of several on Just Security examining The Internationalists: How a Radical Plan to Outlaw War Remade the World, written by Just Security editorial board member Oona Hathaway and her colleague Scott Shapiro.)

Oona Hathaway and Scott Shapiro make the case in their new book, The Internationalists, that the often-derided and long-forgotten 1928 Paris Peace Pact was in fact a watershed in contemporary world politics. What came before 1928 was, according to the authors, the “Old World Order,” where states resolved their disputes—whether territorial, economic, political, or legal—by going to war. This meant war was wrapped up in, and justified by, just about everything, including even the failure to pay debts, trade disputes, and treaty violations. And many of these wars weren’t small or insignificant. They were big, brutally violent, and frequent by today’s standards. They decimated generations and saw large swaths of territory repeatedly change hands.

Following 1928, however, this state of affairs started to change and, with the Paris Peace Pact, states entered an extended teething period, during which the authors argue, a “New World Order” emerged. While this transition period included, notably, World War II, which involved dozens of states and resulted in millions of deaths, Hathaway and Shapiro make the case that war slowly faded from center-stage. It was no longer a go-to tool for states to resolve international disputes and advance their national policies. War became—relatively speaking—a vestige of international law and relations, reserved only for the most extraordinary situations, namely for the narrow purpose of self-defense.

Whether the pact was a catalyst for how states came to re-organize themselves anew, or whether it less impressively reflected a new world order that we were already moving toward is up for debate. The authors try to convince their readers of the former, but agreeing or disagreeing with their conclusion shouldn’t consume all of our attention. Doing so would be a disservice to one of the book’s greatest achievements, which is that it provides ample tinder for deep examinations of a host of other issues.

One of those issues has to do with how, if at all, international law has changed over time to address the enormous amounts of violence that has through history taken place between states and non-state armed groups. The book barely touches on this issue but, to be fair, this is excusable since the Paris Peace Pact didn’t deal with it either. Nonetheless, the book exposes concerns we should all have about the manner in which some states are seeking to re-shape various parts of international law to manage the threats that non-state armed groups pose, and especially those operating in foreign territories.

What Hathaway and Shapiro shows is that international law is, by its nature, an intertwined body of different legal systems heavily based on common understandings of states’ motivations and behaviors. International law is also propped-up by the notion of reciprocity and legal equality among sovereigns. Of course, not all these common understandings turn out to be accurate, not all states behave the same, and there are so many competing rules of international law that in 2000 the International Law Commission felt the need to address the risks arising from the diversification and expansion of international law. But, there nonetheless remains a continuity in international law that in practical terms means that when states start to pull on one of its threads too hard, or in the wrong direction, they risk disrupting other parts of international law in negative ways, often by upsetting the political and economic environments that international law both shapes and upon which it balances itself. Hathaway and Shapiro demonstrate this throughout their book, for example by describing the Old World Order as a system where rules on inter-state cooperation and commerce (which allowed states to benefit from one-another) respected the rules of neutrality (which insulated states from war so long as they didn’t favor one of the warring sides). The authors warn us: “You can’t just yank a thread from a delicate fabric.”

When this advice isn’t followed, the fabric of international law weakens and, at its worst, will tear. It is also troubling that a weakened system of international law—one that loses continuity in its logic and rationale—won’t be able to stand-up to states that commit the most overt of violations. Grotius from the Old World Order (follow the hyperlink, but I highly recommend the book’s first chapter Hugo the Great) and Lauterpacht from the New World Order similarly understood this; and it’s what made them such skillful international lawyers.

Today, several states are responding to the threats that non-state armed groups pose by doing exactly what Hathaway and Shapiro’s book advises against: yanking and pulling at important threads of international law without due regard for the system as a whole. We see this when states advocate for the use of an “unwilling or unable” test and for a “more flexible understanding” of when, in the face of an imminent armed attack by a non-state armed group, they can use force to defend themselves. States claim to do so in the name of increasing their ability to protect themselves. But the novel interpretations they offer have the unavoidable side-effect of replacing narrowly and carefully tailored rules of self-defense that protect today’s international order with more expansive and vague sets of rules that increase, not decrease, potential for inter-state disputes—and potentially inter-state conflicts—over whether the aforementioned standards were applied appropriately.

We also see the United States unraveling international law when it cherry-picks which rules of international humanitarian law (IHL), also known as the law of armed conflict, it wants to apply to its conflict-related detention practices to get its preferred policy result. In doing so, the United States is undercutting IHL’s overall coherence and giving cover for other states to follow suit, weakening the law. The same goes for the concepts of “neutrality” and “co-belligerency,” which provided important rules for states to follow in international armed conflicts (i.e., armed conflicts between two or more states) in the pre-UN Charter days, but which the United States haphazardly misapplied to dramatically expand who it can target (and where it can target them) in the context of today’s non-international armed conflicts (e.g., an armed conflict between a state and a non-state armed group). The Security Council is also guilty of, for lack of a better term, disorganizing international law. Since 2001, it has been in such a state of counter-terrorism zeal that it has pushed states to implement domestic laws that have crowded out and undermined basic principles of IHL and international human rights law.

Throughout history, states have regularly put forth competing interpretations of international law, but it’s still important to ask why all this is happening now at what appears to be an accelerated pace. One answer might be that some government lawyers dealing with international law just don’t take it all that seriously, or ignore the consequences that come from damaging its coherence. Others might care deeply, but the unyielding political pressures that require them to come up with short-term responses simply don’t give them the time to step back and reflect on the logical long-term implications of their novel interpretations.

An additional explanation that Hathaway and Shapiro’s book might prompt us to think more about is this: States feel free to advance novel interpretations of international law to address non-state armed groups because ordinary safeguards, which would otherwise be intolerant of such one-sided interpretations, such as reciprocity and equality among sovereigns, are largely absent. Non-state armed groups simply don’t exist on the same legal level as states and don’t have a seat at international law’s bargaining table. Moreover, states that might ordinarily counter legal interpretations that are out of sync with the larger international legal system often appear too bashful to express opposition. (See, for example, this new piece by Anne Peters). With these factors combined, it’s very easy and tempting for states to think that their novel self-serving interpretations provide them mostly with important advantages, and at minimal cost. Indeed, in this environment it’s also easy for us to mistakenly see many of these new interpretations of international law as mere re-adjustments rather than as actual legal transgressions.

Casual reinterpretation of international law is, however, naïve. Hathaway and Shapiro’s book tells us that when you mess with one part of international law, you’re very likely going to end up messing with other parts of it, and that’s a dangerous thing to do because it tampers with a system of rules that provide a rational equilibrium for inter-state engagement across the globe. This doesn’t mean states need to succumb to static interpretations of international law. But it means that when they start looking to solve their problems with international law in new ways, they need to take a broad and deep look at the downstream implications of their actions.

Image: Heavy smoke from burning oil wells set on fire by fleeing ISIS members is seen blanketing homes on November 9, 2016 in Al Qayyarah, Iraq. (Chris McGrath/Getty Images)