On a dull Monday morning, an aircraft took off from Liverpool bound for the naval air station at Eglinton, Co Londonderry. Conditions forced the pilot to fly around the coast of Northern Ireland to his destination, where his VIP passenger had a rendezvous with history.
It was 75 years ago today - Monday, May 14, 1945. The VIP passenger was Admiral Sir Max Horton. From November 1942, he had been in charge of the battle against Germany's submarine fleet, the U-bootwaffe.
From Eglinton, Horton was driven the short distance to Lisahally and the wharf built for the Royal Navy's Londonderry Escort Force, the ships in the frontline of the U-boat war.
Sailing up Lough Foyle to enter the river and berth at Lisahally was a token flotilla of eight U-boats. Manned by skeleton Kriegsmarine crews under armed Royal Navy guard, they would make the official surrender of Germany's submarines.
With the U-boats sailed three warships. The destroyer HMS Hesperus, no stranger to the Foyle, represented the Royal Navy. HMCS Thetford Mines, a frigate, was one of many Derry-based Canadian ships. Finally, the USS Robert I Paine, a destroyer-escort, represented the United States Navy.
The three warships represented the principal elements of the alliance that had defeated the U-boats. Since September 3, 1939, the first day of the war for the UK, U-boats had taken a heavy toll.
On that September Sunday, the liner Athenia had been sunk off the north coast of Ireland. And on May 7, 1945, just before VE Day, two merchant ships had been sunk in the Firth of Forth by a U-boat - after the German surrender.
Why had Horton arranged the surrender at Lisahally? The answer was simple. In the Allied campaign to defeat the U-boats there was no base more important than the Royal Navy's Londonderry base.
The farthest west in the UK that escort ships could be based, the Foyle became strategically important once U-boats began operating from the French Atlantic ports.
With convoys re-routed around Ireland's north coast, the fulcrum of war had placed the city on the Foyle at the centre of the Battle of the Atlantic, the longest campaign of the Second World War.
From late 1940, the base had been built up and, as HMS Ferret, was home to almost 150 oceangoing warships. The numbers of such ships based at Liverpool, Greenock and, from 1943, Belfast was under 100.
The Londonderry base also provided critical training facilities, plus air bases. From HMS Shrike, the naval air station at Maydown, flew the Swordfight bombers that, from mid-1943, ensured that all convoys had air cover right across the Atlantic.
In short, the most important base in the battle had been Londonderry. And that was why Horton chose the city over any other major naval base to mark the U-boats' formal surrender.
For almost five years, escort groups - six to 10 ships - had sailed from the Foyle to shepherd convoys either way across the Atlantic. Sailing in all weathers, they braved the very worst conditions.
Occasionally, a voyage passed almost without incident. At other times, there was frenetic activity as U-boats sought to sink merchant ships, especially the tankers.
Initially, with the tactical advantage with the U-boats, the toll of merchant shipping was high. However, as more escorts became available, tactical training improved, and new weaponry and technology was developed, the advantage swung to the escort ships and aircraft.
Another critical factor was the Royal Navy's intelligence advantage. While Bletchley Park decrypted Enigma messages, this wasn't the Allies' sole intelligence.
All German surface ship and submarine captains reported regularly by radio to their HQs. Their messages were intercepted and the transmitting locations identified.
The Germans didn't believe Enigma could be decrypted. Neither did they believe that the "English" could miniaturise high-frequency radio-direction-finding equipment and fit it on small ships.
But they had: detecting such signals led to the destruction of many U-boats. Others abandoned planned attacks when they realised that escort ships were closing on them.
All those developments - training, tactics, weaponry, technology and signals intelligence, plus aircraft flying from aircraft carriers - coalesced in May 1943.
Weeks before it looked as if the U-boat commander, Admiral Karl Dönitz, was "torpedoing his way to victory".
Such was the loss of merchant ships from late-December 1942 to April 1943 that it seemed that the U-boats would win the Battle of the Atlantic.
During May everything changed, firstly with Convoy HX237, escorted by the Foyle-based Canadian 2nd Escort Group. Lieutenant Commander Evelyn Chavasse, from Co Waterford, commanding the group, brought the convoy safely to the UK.
En route, supported by Swordfish from an aircraft carrier and land-based aircraft, he disrupted a U-boat pack. Two U-boats were sunk by escorts and others by aircraft.
Commander Peter Gretton, 7th Escort Group, was escorting Convoy SC130 and also faced packs of U-boats. He also brought his convoy safely home, with four U-boats sunk. The German dead included Admiral Dönitz's son, Peter. Once again, teamwork had prevailed.
Also operating in the Londonderry Escort Force was Captain Donald Macintyre, commanding 2nd Escort Group from HMS Hesperus. Macintyre, one of the leading anti-submarine "aces", was later a historian of the Battle of the Atlantic. Thus began the defeat of the U-boats, finally made clear in that surrender ceremony at Lisahally - the only German surrender to take place in Northern Ireland.
Alongside Horton at Lisahally that day was Colonel Dan Bryan, director of Irish intelligence, and Sir Basil Brooke, Prime Minister of Northern Ireland. Bryan's presence reflected a little-known aspect of the Allied victory - the support and assistance provided by neutral Ireland.
Not only were coast-watching signals transmitted in plain speech, allowing the UK authorities to intercept them, but the Irish Government had permitted Allied aircraft to overfly Donegal, at Ballyshannon and over Inishowen, as well as permitting the stationing of an air-sea-rescue trawler, HMT Robert Hastie, in Killybegs. And, for most of the war, a tanker lay off Moville to refuel escort ships.
Richard Doherty is the author of Churchill's Greatest Fear: The Battle of the Atlantic 3 September 1939 to 7 May 1945