After a journey almost ended by a German aircraft attack, four American intelligence officers arrived late one night in February 1941 at the large, gloomy mansion in the English countryside that was home to British spy chiefs and code breakers.

Ushered inside, they began a trade in top-secret information that continues to this day, after being welcomed with a glass of sherry.

Seventy-five years later, that toast at Britain’s wartime code-breaking center, known as Bletchley Park, is being held up as the start of one of the world’s greatest intelligence-sharing relationships, one that remains at the center of the two countries’ security ties.