Traditional international relations theory operates within Kenneth Waltz’s “third image,” in which systemic anarchy and the security dilemma are treated as structural constants, and John Mearsheimer’s offensive realism, which sees great-power competition as inescapable. Charles Tilly’s account of the territorial state’s extractive superiority and Hendrik Spruyt’s analysis of institutional selection explain why the sovereign territorial state outcompeted city-states and leagues after the feudal era, locking in a system of functionally similar, territorially defined units capable of waging sustained war. This article contends that these structural causes of war are not universal but contingent on Westphalian conditions of contiguous territory, survivable environments, and unconstrained resource extraction. The 1967 Outer Space Treaty, whose Article II prohibition on “national appropriation” by claim of sovereignty, use, occupation, or any other means has achieved near-jus cogens status, combined with the lethal fourth-image environment of Mars, renders territorial sovereignty impossible and selects instead for non-territorial city-states. Only functional sovereignty—jurisdiction over personnel, objects, and operations within localized facilities, as conceptualized by Nicholas Katzenbach—remains permissible under the Outer Space Treaty and Article VIII. Territorial claims constitute prohibited appropriation by occupation or use; the sole lawful delimitations are activity-based, temporary safety zones derived from operational necessity. The Artemis Accords operationalize these principles through soft-law deconfliction and coordination mechanisms. These norms subsequently harden into the constitutional architecture of the Charter of the United Martian States, which institutionalizes enumerated powers, a minimal Union defense capacity, an unamendable Guarantee Clause, and a rights floor. In this reversed Spruytian selection process, the non-territorial city-state becomes the dominant institutional form. Michael Doyle’s democratic peace is thereby structuralized rather than contingent. Mutual vulnerability, radical transparency, and legal firewalls suppress the security dilemma, transforming anarchy into a pacified, republican planetary order. Mars thus functions as a natural laboratory in which outer space law acts as an independent variable that reshapes units, sovereignty, and the prospects for war and peace, forcing terrestrial international relations theory to confront the limits of its Westphalian assumptions.