How International Law Based on Treaties and Customs Shapes Global Order

Published 14 September 2026

The foundation of international law rests not on a single legislative body but on a delicate balance between two pillars: treaties and customs. Unlike domestic legal systems, where statutes and constitutions provide clear, enforceable rules, international law operates through voluntary agreements and long-standing practices. This dual system—rooted in centuries of state interactions—creates a web of obligations that govern everything from trade disputes to human rights. Yet, its effectiveness hinges on a paradox: states adhere to these rules not because of coercion, but because their collective interests demand it.

Consider the Law of the Sea, a cornerstone of maritime governance. How is international law based on treaties and customs here? The 1982 UN Convention on the Law of the Sea (UNCLOS) codified centuries of customary practices—such as freedom of navigation—into binding rules. But even without the treaty, many of its principles (like territorial waters extending 12 nautical miles) were already accepted norms. This interplay between formal agreements and informal traditions illustrates why international law remains both flexible and fragile. One misstep—such as a state unilaterally ignoring a treaty—can unravel decades of diplomatic consensus.

The tension between treaties and customs is most visible in crises. When Russia annexed Crimea in 2014, it cited historical claims (custom) while Western powers invoked the Budapest Memorandum (treaty). The clash revealed how how international law based on treaties and customs can be weaponized—or neutralized—depending on a state’s interpretation of legitimacy. The absence of a global police force means compliance is often a matter of perception, not enforcement. Understanding this dynamic is key to grasping why international law evolves as much through precedent as through ink on paper.

The Complete Overview of How International Law Based on Treaties and Customs Functions

International law’s dual nature—how international law based on treaties and customs operates—emerges from a pragmatic necessity. Treaties provide specificity: they define rights, obligations, and dispute-resolution mechanisms in black-and-white terms. Customary law, meanwhile, fills the gaps with principles that states accept as binding over time. Together, they form a hybrid system where legal certainty coexists with adaptability. For instance, the Genocide Convention (1948) outlawed atrocities in treaty form, but the concept of "crimes against humanity" had already taken root in post-WWI tribunals, reflecting customary condemnation of mass violence.

Yet this system is not without flaws. Treaties require ratification, meaning a single holdout (e.g., the U.S. never ratified the Rome Statute for the ICC) can undermine global consensus. Customary law, while influential, lacks precise definitions—leading to disputes over what constitutes a "custom" (e.g., the Nicaragua v. United States case, where the ICJ ruled that U.S. mining of Nicaraguan harbors violated customary international law). The interplay between the two thus creates a perpetual negotiation: when treaties fail to address new challenges (cyber warfare, climate change), customary norms often step in to guide behavior—until a new treaty can be drafted.

Historical Background and Evolution

The origins of how international law based on treaties and customs trace back to ancient civilizations, where city-states and empires relied on mutual agreements to regulate trade and warfare. The Treaty of Tordesillas (1494), dividing the New World between Spain and Portugal, was one of the first formal attempts to codify territorial claims. But it was the Peace of Westphalia (1648) that cemented the modern concept of state sovereignty—a principle later reinforced by treaties like the Vienna Convention on the Law of Treaties (1969). Meanwhile, customary law evolved through repeated state practice, such as the Hague Conventions (1899/1907), which established rules of war based on long-standing military traditions.

The 20th century marked a turning point. The horrors of World War II led to the creation of the United Nations Charter (1945), which explicitly recognized both treaties and customary law as sources of international obligations. The International Court of Justice (ICJ) further solidified this duality by ruling in North Sea Continental Shelf (1969) that customary law could emerge even without a treaty if states acted consistently and opinio juris (a belief in legal obligation) was present. Today, the interplay between the two remains dynamic: while treaties like the Paris Agreement (2015) set binding climate targets, customary principles (e.g., the precautionary principle) guide actions in areas where treaties are absent.

Core Mechanisms: How It Works

The machinery of how international law based on treaties and customs functions through a mix of formal and informal processes. Treaties are negotiated, signed, and ratified through diplomatic channels, often with provisions for amendment or withdrawal (e.g., the Iran Nuclear Deal’s "snapback" mechanism). Customary law, however, develops organically: states must demonstrate consistent behavior (e.g., prohibiting torture) and an acceptance that this behavior is legally required. The ICJ’s Asylum Case (1950) set the standard for proving custom, requiring evidence of general practice and opinio juris.