City in Germany on the Havel River that hosted a 1945 meeting of the principal Allies in World War II (7)
I believe the answer is:
potsdam
(Other definitions for potsdam that I've seen before include "1945 conference location" , "WW two 1945 conference; stop mad (anag.)" , "City near Berlin, site of Sanssouci palace" , "conference city" , "Site of WW2 conference" .)