Definition
The law of arms is the body of rules and customs governing the conduct of war between sovereigns and nations — including the proclamation of hostilities, the making and observance of leagues and truces, the treatment of combatants and prisoners, and the punishment of military offenders. It operated as a species of international customary law, binding not by the authority of any superior tribunal but by the mutual recognition of sovereign princes who stood in no subordinate relationship to one another and therefore had no ordinary judge to resolve their disputes.
The term is largely historical. Modern usage has consolidated its subject matter under the law of war, international humanitarian law, and the law of armed conflict. Researchers encountering "law of arms" in pre-twentieth-century legal sources should treat it as the predecessor vocabulary for these fields.
Common Confusion
LAW OF ARMS vs. LAW OF WAR: The historical dictionaries are explicit that these terms describe the same substantive body of law, with "law of war" emerging as the dominant modern label. The distinction is purely one of era and idiom, not substance. Researchers should not read "law of arms" as a narrower or different doctrine from "law of war" when interpreting historical texts — the shift is terminological, not conceptual.
LAW OF ARMS vs. RIGHT TO BEAR ARMS: Modern readers may instinctively associate "arms" with Second Amendment doctrine or individual weapons rights. That association is entirely inapposite here. The law of arms, as a classical legal category, concerns the conduct of war between sovereigns, not individual rights to possess weapons. The Second Amendment constitutional framework and its case law are a separate body of law with no historical connection to this term.
Why It Matters in Research
The primary research trap with this term is anachronism in both directions. Working backward from modern sources, a researcher might miss it entirely — it does not appear in contemporary statutory codes or treatises under this name. Working forward from historical sources, a researcher must recognize that legal authorities from the sixteenth through nineteenth centuries used "law of arms," "law of war," and occasionally "jus militare" or "lex armorum" interchangeably, often within the same treatise.
In the historical Law Mind corpus, this term surfaces most reliably in English legal dictionaries drawing on Cowell's Interpreter (1607) and Blount's Nomo-Lexikon (1670), which were themselves synthesizing continental and customary sources. Bouvier's entry is notably more theoretically developed than Black's or Burrill's, emphasizing the jurisdictional rationale — that sovereign princes lacking a common superior required a horizontal law of mutual obligation — which maps directly onto what later became the doctrinal foundation of public international law.
Researchers working on the history of international law, the law of war, military jurisdiction, or prize law should treat "law of arms" as the entry point for tracing how these doctrines were understood before the codification movements of the late nineteenth and early twentieth centuries (the Hague Conventions, the Geneva framework). The conceptual architecture in Bouvier's entry — sovereign equality, no superior judge, mutual obligation through custom — remains analytically live in contemporary international law scholarship, even though the vocabulary has changed entirely.
The term also appears in heraldic law contexts in English legal history, where "law of arms" sometimes referred to the jurisdiction of the Court of Chivalry over armorial bearings and questions of gentility. This is a distinct usage and should not be confused with the international law meaning that dominates in American legal dictionaries. Historical sources, particularly English ones, may use the term in either sense without disambiguation.
Historical Dictionary Support
Black's and Burrill's entries are nearly identical, both drawing directly on Cowell and Blount, and both explicitly redirect the reader to "law of war" as the current equivalent. This parallelism reflects the common practice of nineteenth-century American legal dictionaries in condensing and relaying English lexicographical predecessors rather than independently synthesizing doctrine.
Bouvier's entry is the outlier in a useful way. It adds the theoretical grounding absent from Black's and Burrill's — the rationale that the law of arms was necessary precisely because sovereigns have no superior or ordinary judge. This is the classical Vattelian premise of international law, and Bouvier's inclusion of it (citing "Treat. Laws 57" and Jacob's Law Dictionary) signals that American legal thinkers of his period understood the law of arms not merely as a set of procedural military rules but as a jurisprudential response to the problem of anarchy among sovereigns.
What all three historical sources understate is the scope of this body of law as it was understood in its prime: it encompassed not only battlefield conduct but the entire law of nations as applied to armed conflict, including neutrality, blockade, contraband, and the rights of non-combatants — subjects that nineteenth-century international lawyers were actively elaborating under new names even as these dictionary entries were written.
Encyclopedia Cross-Reference
The Law Mind Constitutional Law Encyclopedia: The Right to Bear Arms and State Militia (Historical Context of the Second Amendment) — for background on how "arms" vocabulary evolved in constitutional law, which is a distinct but potentially confusing parallel track.