2004 Iranian seizure of Royal Navy personnel
On 21 June 2004, in the Shatt al-Arab (Arvand Rud in Persian) waterway, six Royal Marines and two Royal Navy sailors were captured by Iran. The British servicemen were seized while training Iraqi river patrol personnel after Iran said they had strayed to the Iranian side of the waterway.
Source: Wikipedia — 2004 Iranian seizure of Royal Navy personnel (CC BY-SA 4.0)