Royal United Services Institute
London defence think tank; its Arrow-3 depletion analysis shaped coverage of the Iran war.
Last refreshed: 31 March 2026
If RUSI is right on Arrow-3 exhaustion, what stops Iran's next major missile salvo?
Latest on Royal United Services Institute
- What did RUSI say about Arrow-3 missiles running out in the Iran war?
- RUSI projected Israel's Arrow-3 interceptor stocks were 81.33% depleted by 26 March 2026 and would be fully exhausted by the end of March, with THAAD facing similar depletion within one month.Source: RUSI
- How many Arrow-3 interceptors does Israel have left?
- RUSI estimated fewer than one in five of Israel's pre-war Arrow-3 interceptors remained by 26 March 2026, with full exhaustion projected by end of month.Source: RUSI
- What is the Royal United Services Institute?
- RUSI is the world's oldest defence and security think tank, founded in 1831, based in Whitehall, London. It conducts independent research on military, security, and geopolitical issues.
- Who runs RUSI and when was it founded?
- RUSI was founded in 1831 by the first Duke of Wellington. Its Director-General is Rachel Ellehuus, who took up the post in January 2025.Source: RUSI
- RUSI Iran war analysis 2026?
- RUSI's most influential 2026 Iran war publication modelled interceptor depletion rates, finding the US-Israel Coalition expended 11,294 munitions in the first 16 days at a combined cost of $26 billion.Source: RUSI
- How does RUSI compare with Chatham House or IISS on defence analysis?
- All three are independent London-based think tanks. RUSI focuses on defence and security specifically; Chatham House covers international affairs more broadly; IISS specialises in military balance and arms control.
- Is RUSI funded by the UK government?
- RUSI receives no core government grant. It is funded through membership subscriptions, research contracts, and donations from public and private sources, including the UK FCDO.Source: RUSI
Background
RUSI became central to the Iran conflict in March 2026 when its researchers published the most-cited assessment of Israel's missile-defence crisis: Arrow-3 stocks were 81.33% depleted by 26 March and projected to reach zero by month's end. THAAD stocks faced similar pressure within weeks. The analysis reshaped political debate in Washington and London over the timeline for a Ceasefire.
The Royal United Services Institute, founded in 1831 by the first Duke of Wellington, is the world's oldest defence and security think tank. Based in Whitehall, London, it operates independently of government, funding itself through membership, research contracts, and donations from a range of public and private sources including the UK Foreign, Commonwealth and Development Office. Its Director-General is Rachel Ellehuus, who took the post in January 2025. Research focuses on military sciences, international security, terrorism, cyber threats, and nuclear proliferation.
RUSI's work carries particular weight in UK policy circles because of its proximity to the Ministry of Defence and Whitehall. Its Arrow-3 findings circulated widely in Western capitals and contributed to US signalling of openness to diplomatic off-ramps in late March 2026.
