Dunkirk
Dunkirk was located in northern France, located along the English Channel from England. Its name was a byword for a historical event that by the turn of the 25th century, people had started to forget.

In 2399, Retired Admiral Jean-Luc Picard used Dunkirk as an analogy for describing the mass evacuation fleet proposed for the rescue of nine hundred million Romulans from the Romulus system before its destruction in 2387. He accused a reporter of having no idea what Dunkirk was. ()

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Background information
Considering the context, the event alluded to was almost certainly the  during World War II. Though born one month after the actual event, Picard performer Patrick Stewart still has personal connections to the historical Dunkirk evacuation as he had discovered in his 2012 episode of the British Who Do You Think You Are? television series (Season 9, Episode 3); his father Alfred was among the troops evacuated from there, contracting  in the process, which, being undiagnosed at the time, left its mark on Stewart's childhood. This discovery led directly to Stewart's activism for the British charity "Combat Stress" which helps veterans deal with PTSD. 

Ronald D. Moore commented that historical parallels brought up during the story discussions during the writing of  included the evacuation of  by the  in 1940 as an inspiration.

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Earth geography