International law shapes state behavior through formal legal obligations and shared norms, even though centralized enforcement mechanisms are weak. States comply because of reputational concerns (future punishment), domestic politics (courts, legislatures constrain executives), reciprocity expectations (others will obey if you do), and legitimate authority (belief in the rightfulness of rules).
Study specific treaty regimes (Nonproliferation Treaty, Law of the Sea Convention, Paris Climate Agreement) and measure compliance rates. Investigate why some states comply while others violate; test whether enforcement mechanisms actually drive behavior or whether reputational and domestic mechanisms suffice.
Your study of international relations established the anarchic structure of the international system: there is no world government, no global court with compulsory jurisdiction over powerful states, no police force that can compel the United States or China to obey rules they prefer to ignore. Given this, a natural question is: why would international law matter at all? The realist answer is essentially that it doesn't — states follow rules when they happen to serve their interests and defect when they don't. But the empirical record is more interesting than this suggests. Most international law is complied with most of the time, by most states, even in the absence of enforcement. Understanding why is the central puzzle of this topic.
The first compliance mechanism is reputation and reciprocity. A state that defects from an international agreement today signals to every other state that it is an unreliable treaty partner. This matters because international cooperation is an iterated game: states interact repeatedly, and a reputation for defection raises the cost of every future agreement you try to negotiate. Other states will demand more verification, offer less favorable terms, or refuse to partner with you altogether. The shadow of the future — anticipated consequences of today's behavior on future interactions — creates an incentive to comply even when today's immediate payoff favors defection. This is why even powerful states care about their international reputation: the US, Russia, and China all invest significantly in presenting themselves as rule-followers (even when they aren't), because the reputation for reliability is a diplomatic asset.
The second mechanism is domestic politics. The realist model treats the state as a unitary actor with a single national interest. But states are governed by institutions — executives, legislatures, courts, bureaucracies — and international law enters domestic politics in ways that can constrain executive behavior. In liberal democracies, ratified treaties become part of domestic law. Courts can rule executive actions in violation of treaty obligations. Legislatures can condition funding on treaty compliance. Advocacy groups use international legal norms to mobilize domestic political pressure. This "liberal" model of compliance — running through domestic institutions rather than through external enforcement — explains why democracies often have higher compliance rates than authoritarian states: their internal institutional architecture provides more entry points for international legal norms to shape behavior.
The third mechanism is legitimacy — the belief that a rule is rightful and deserves compliance regardless of whether you can be punished for violation. This is perhaps the most sociologically interesting mechanism because it operates even in the absence of reputational or domestic pressure. Legal scholars and constructivist IR theorists argue that states that participate in the development of international legal rules internalize those norms — they genuinely come to believe that the rule is right and that compliance is a matter of good international citizenship. This explains why states follow elaborate diplomatic protocols, honor foreign sovereign immunity, and refrain from actions that are technically legal but widely seen as norm violations: they are acting on the basis of legitimacy, not just calculation.
The practical implication is that compliance is a variable, not a binary. Different treaty regimes achieve different compliance rates. The Montreal Protocol (ozone depletion) achieved near-universal compliance within two decades. The Nuclear Nonproliferation Treaty shows mixed compliance with a small number of persistent defectors. The Paris Climate Agreement has varied compliance linked to domestic politics in signatory states. Analyzing a compliance record requires asking: what enforcement mechanisms exist? What are the reputational stakes? What domestic political dynamics affect the regime? What is the perceived legitimacy of the underlying norm? The answer typically involves all four mechanisms operating simultaneously, with different weights in different cases.
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