The Twelve-Day War: Israel and US Strike Iran's Nuclear Programme to Ceasefire

The Israeli decision to launch the Twelve-Day War on June 13, 2025 emerged from a convergence of intelligence assessments and political conditions that the Netanyahu government judged to be closing. By mid-2025, Israeli and Western intelligence services assessed that Iran had accumulated sufficient highly enriched uranium to produce multiple nuclear devices, and that the technical gap between enriched material and deliverable weapons had narrowed to a period measured in weeks rather than months. The diplomatic track that had been pursued through European intermediaries since the collapse of the JCPOA had produced no verifiable constraints on enrichment, and the Israeli government had concluded — after the October 2024 exchange, in which Israeli aircraft had struck inside Iran proper for the first time and received a measured rather than overwhelming response — that a window existed for a more comprehensive degradation campaign before Iran had crossed what Israeli planners termed "the final threshold." The June 13 strikes were large-scale and simultaneous across multiple target categories: nuclear enrichment infrastructure at Natanz, Fordow, and the Isfahan conversion facility; ballistic missile production and storage facilities whose continued operation had enabled the October 2024 barrage; air defence networks protecting nuclear sites; and IRGC command and logistics infrastructure supporting the nuclear programme. The operation used bunker-penetrating munitions developed specifically for hardened underground facilities and drew on years of accumulated intelligence about facility layouts and hardening specifications. Iran responded with successive ballistic missile salvos targeting Israeli cities and military bases, producing civilian casualties in Israel and triggering the Iron Dome and Arrow systems to their highest operational tempo in the conflict's first days.

The United States entered the war directly on June 22, nine days into the Israeli campaign, when President Trump authorised strikes on three Iranian nuclear facilities that Israel's munitions had been unable to fully destroy: the deeply buried Fordow enrichment hall, a secondary enrichment facility at Natanz's underground cluster, and a weaponisation research site whose existence had been known to US intelligence but not previously confirmed publicly. The US strikes used the GBU-57 Massive Ordnance Penetrator — the most powerful conventional bomb in the US arsenal, designed specifically to penetrate reinforced underground structures — delivered from B-2 stealth bombers operating from Diego Garcia. The decision to deploy the MOP represented the most significant direct US military action against Iran since the 1988 Operation Praying Mantis, and it crossed a threshold that every prior US administration since Reagan had avoided: the direct destruction of Iranian sovereign infrastructure by American military force. Iran continued firing missiles at Israel and began targeting US assets in the region — air bases in Qatar, Kuwait, and Bahrain — in response to the direct American intervention, escalating the conflict to a three-actor exchange. Iranian missile salvos during the nine days of the Israeli campaign and the subsequent three days of US participation demonstrated that strike damage to launch infrastructure had degraded Iran's capacity but had not eliminated it: missiles continued to arrive, though at declining frequency and with declining accuracy as command networks were disrupted.

The ceasefire brokered by the United States on June 24, 2025 ended eleven days of direct military exchange between Israel, the US, and Iran. Its terms were asymmetric: Iran agreed to suspend enrichment activities at all damaged facilities pending an international inspection process; Israel agreed to cease air operations; the United States committed to facilitating the inspection mechanism. The ceasefire's most significant limitation was that it did not resolve the core question it had been constructed to address — whether Iran's nuclear programme had been permanently set back or merely damaged and suspended. Western governments and Israeli intelligence assessments diverged on how long Iran would require to reconstitute enrichment capacity: estimates ranged from eighteen months to several years depending on assumptions about equipment replacement, international sanctions, and Iranian political will. Iran's Supreme Leader Ali Khamenei, emerging publicly after days of silence during the strikes, stated that Iran's nuclear rights under international law remained intact and that any agreement to suspend enrichment was conditional on the lifting of sanctions — a formulation that accepted a temporary pause while rejecting permanent dismantlement. The post-war debate among Western policymakers centred on whether the Twelve-Day War had bought time or merely accelerated Iran's motivation to cross the nuclear threshold permanently before a second campaign could prevent it. That debate remained unresolved when Operation Epic Fury was launched on February 28, 2026 — a second, far larger campaign premised on the assessment that the first had not achieved its strategic objective and that a more comprehensive operation, this time including the decapitation of the supreme leadership, was required to produce a durable outcome. The Twelve-Day War also reshaped the domestic political landscape inside Iran: Khamenei's survival had required him to accept a temporary enrichment suspension that IRGC hardliners framed as capitulation under fire, deepening the internal tension between the civilian government's diplomatic instincts and the IRGC's maximalist refusal to accept any arrangement that permanently constrained Iran's nuclear programme — a tension that became the defining structural conflict of the months between the June 2025 ceasefire and the February 2026 campaign that ended Khamenei's life.