The present study explains why the United Nations Security Council (UNSC) during the Gaza crisis (October 2023 to late 2025) proved unable to translate the concept of "collective security" into clear, binding, and enforceable constraints, and then examines the implications of this institutional pattern for the strategic recalibration of the Islamic Republic of Iran's regional policy through the lens of Offensive Realism. In the first part, the article draws on Stephen Krasner's theory of international regimes to conceptualize the Security Council as a regime-like institutional arrangement composed of principles, norms, rules, and decision-making procedures operating within (and conditioned by) the global distribution of power. On this view, the Council's limited efficacy is primarily structural rather than merely managerial. The argument specifies three principal channels through which U.S. influence is exercised: (1) the observable pattern of veto use; (2) ex ante shaping of draft resolutions under the shadow of a credible veto threat, which narrows the bargaining space and shifts texts toward what is "adoptable"; and (3) procedural control over agenda management and the drafting process, through which the sequencing of options and the framing of outcomes can be steered toward lower-cost commitments. To assess UNSC performance in an analytically tractable way, the study distinguishes between output effectiveness and outcome effectiveness. Output effectiveness refers to the Council's ability to produce authoritative decisions (especially on high-salience issues). In contrast, outcome effectiveness is operationalized through a proxy: the gap between the Council's stated demands and the continuing conflict dynamics and humanitarian emergency as reflected in United Nations humanitarian reporting. The findings indicate a recurrent pattern of selectivity: the Council adopted resolutions focused on humanitarian access and certain phased or sequenced frameworks, yet repeatedly failed to secure an immediate, unconditional cessation of hostilities. This failure was closely associated with U.S. vetoes and bargaining processes that converged around Washington's preferences, thereby sustaining a persistent divergence between humanitarian norms and the operational rules capable of constraining conduct. In the second part, the article argues that, from an Offensive Realist standpoint, structurally selective enforcement generates incentives for Iran to reduce reliance on institutional restraint and to pursue a calibrated maximization of deterrent and diplomatic leverage outside—and alongside—the UNSC framework. This recalibration is theorized as a form of "calibrated revisionism": not a wholesale attempt to overturn the international order, but a selective adjustment strategy aimed at mitigating asymmetric constraints and raising the costs of coercive alignment against Iran. In policy terms, the study identifies a multi-track package that includes networked deterrence, denial capabilities, wedge-driving diplomacy (alignment engineering to complicate counter-coalition formation), and legitimacy contestation in venues not subject to veto control. The article's overarching contribution is a two-level causal framework that links U.S. regime-maintenance dynamics to the persistence of a norm–rule gap at the global level and, at the regional level, to Iran's strategic adaptation in coalition management and leverage-building under conditions of institutional selectivity.