In the black of night, our patrol boat skimmed across the Shatt al-Arab waterway, looking out for intruders on the disputed waters.
I was accompanying a Royal Marine patrol as it cruised Iraqi waters looking for suicide bombers trying to attack the two oil platforms that export 90 per cent of the country's oil. The patrol was also hunting smugglers bringing arms and contraband into the country.
Until this point, our only contact with the Iranian Revolutionary Guards had been polite, but stiff, contacts over the radio. On Thursday, when HMS Cornwall spotted an Iranian ship on the Iraqi side of the waterway, she approached to warn them off. The Iranians slunk into the blackness without demur.
All changed dramatically yesterday morning when 15 Royal Marines and Navy personnel, including one woman, approached a Japanese merchant ship suspected of smuggling second-hand cars into the country without paying tax. Suddenly, their inflatables were surrounded by boats of the Revolutionary Guards and they were overpowered and taken into Iranian national waters.
Last night, the British and Iranian governments were locked in a diplomatic row as the dispute escalated. Britain protested that its personnel had been " kidnapped" while Iranian state television insisted that UK service members were "under arrest" for entering Iranian territory.
In London, the Iranian ambassador was called in for a dressing down and told "in no uncertain terms" that the country expected its service personnel and equipment returned.
The capture has inflamed an already tense situation between the two countries over accusations that Iran is fomenting the insurgency in Iraq while defying the world over its suspected nuclear weapons programme.
HMS Cornwall remained at sea off the coast of Iraq last night, those on board all too painfully aware that 15 of their crew had not come back that night and were being held prisoner just miles away across the border by Iran's elite Republican Guard.
Only hours before I had been out on patrol with the group as they chatted to local fishermen as part of a "hearts-and-minds" operation instigated by the British when they took over command of the coalition force off the coast of Iraq three weeks ago.
The Royal Navy frigate's 300 crew stood silently as the voice of their commanding officer, Commander Jeremy Woods, echoed through the corridors and cabins: "This emphasises the seriousness of the task we are doing out here," he said. "I am very confident we will have our people home soon."
Many aboard were trying to remain upbeat, recalling that only three years ago the Iranians captured eight Royal Navy personnel, only to release them after three days. They were accused of spying and had to endure mock executions before finally being released. Their boats were never returned.
The Marines and sailors had been out patrolling near al-Faw peninsula in fast rigid-hull inflatable boats (Ribs) yesterday morning when they spotted the merchant ship. Suspecting that it might be part of the booming smuggling trade, the Royal Navy boarded the ship to speak with its captain and crew.
Lt-Cdr Phil Richardson, the pilot of a Lynx helicopter which had been providing cover, said the crew of the ship appeared co-operative and friendly so he was asked to continue reconnaissance in the rest of the area.
Suddenly, however, the two small Ribs lost contact with HMS Cornwall and the helicopter was sent back to find the crew of the merchant ship, pointing frantically towards the mouth of the Shatt al-Arab river. As the pilot and observer moved closer up the waterway, they spotted Revolutionary Guard fast-attack speedboats mounted with machine guns.
"They were stopped and I could identify the Royal Navy ensign and picked out one of our boats and four of five of the crew, obviously detained, " the pilot said.
Briefly, Lt-Cdr Richardson managed to make contact with the Revolutionary Guards, who said they had arrested the British service members for straying into Iranian waters but that they were safe. The crew were then taken to an Iranian military base.
On Thursday night, only hours before they were taken by the Iranians, the Marines and sailors had been in a jovial mood. They were patrolling waterways as part of a military operation put in place three years ago after suicide bombers in three dhows attacked al-Basra and Khawr al-Amaya oil terminals, killing three US sailors and a coastguard. A three-kilometre exclusion zone had been subsequently thrown up around the terminals.
The hearts-and-minds operation was instigated by the British Commodore, Nick Lambert. As a result, Marines in Ribs often exchange pleasantries with local fishermen and hand out small gifts such as gloves.On Thursday night, the Marines' boats were dwarfed by the giant, rusty hulk of Khawr al-Amaya oil platform as they sped towards the winking lights of fishing dhows bobbing on the horizon. Minutes later, they reached their destination.
The Iraqi and Iranian families who have fished these waters for generations work on the perimeter of the zone, frequently chancing their luck over the exclusion zone before being nosed back by the larger military vessels.
"They are pretty welcoming," one officer explained. "During the day, they can be quite brief because they are working, but at night they are happy to talk."
To prove his point, a row of smiling faces appeared on the deck and gestured to the Navy personnel to come on board.
A round of handshakes and "Salaam Alaykums" criss-crossed the gathering before the Marine officer proceeded to exchange small talk in Arabic with the ship's captain and first mate.
"We feel safe with the [Navy] ships here," one fisherman, Abdul Rahman, explained. "But it is hard to support our families," he added, looking with yearning at the giant oil structures in the distance. " It is better fishing near the platforms, better than here." While the marines inspected the vessel, the officer continued to exchange pleasantries with the fisherman as they showed of their day's catch of shrimp and offered cups of tea.
Last night, Commodore Lambert denied his men had strayed into Iranian waters, insisting they were half a mile inside Iraq around Marakkat Abd Allah. "My immediate concern is for the safety of my people and their safe return," he said. "Everything is being done ... at the highest level of the UK government and by our coalition partners to ensure that this is possible."
Officers said they hoped this was an error of judgement made by a local commander that would be resolved with negotiations. "I hope we find this is a simple misunderstanding at the tactical level," the Commodore said.
While tensions at national level have been strained lately, the British insist that they have maintained a cordial relationship with the Iranians. The border is indistinct and frequently the Iranians stray close to the coalition ships but, until now, they have moved back across the diving line without dispute.
Commodore Lambert said: "We have a healthy, professional respect. We police and patrol our side, and they police and patrol their side."