Over the past five decades, Iran and the United States have made several attempts to ease their mutual hostility and pursue cooperation, most notably through the Joint Comprehensive Plan of Action (JCPOA). However, these intermittent periods of de-escalation and cooperation have never led to lasting reconciliation. This article offers an alternative explanation for the recurring cycle of escalation and de-escalation in tensions between Tehran and Washington. It asks why Iran-US relations have not followed a linear trajectory toward either the consolidation of hostility or normalization, but have instead fluctuated between periods of escalating and easing tensions. The article argues that this fluctuation stems from the two governments’ changing perceptions of the costs and benefits of confrontation and cooperation. Perceptions of hostility and threat from the other side strengthen the inclination toward confrontation, while the rising costs of confrontation and sustained tensions subsequently increase the incentives for de-escalation. Over time, the deepening of mutual hostility has prevented de-escalation from developing beyond a brief and temporary phase, while imposing significant domestic constraints on efforts to normalize bilateral relations. The study adopts neoclassical realism as its theoretical framework and employs process tracing as its research method. Written in the aftermath of the 2026 Iran-US war, the article makes a distinctive contribution by demonstrating how the strategic and policy dilemmas underlying relations between Tehran and Washington have obstructed efforts at de-escalation, allowing renewed and more intense waves of confrontation and conflict to emerge in place of lasting reconciliation.