French port near the Belgian border (7)
I believe the answer is:
dunkirk
(Other definitions for dunkirk that I've seen before include "French port, scene of a 1940 Allied evacuation" , "Withdrawal" , "Crisis where a desperate effort is the only alternative to defeat?" , "Scene of Allied troop evacuation, 1940" , "Scene of the evacuation of British forces in 1940" .)