这个是非限制性定语从句。
which指的就是 the very date of the capitulation 投降的那一天。
你先看主句，就是
From the very day of the capitulation, Paris stood on the watch.
从句就是
by the very day of capitulation Bismark’s prisoners had signed the surrender of France but reserved to themselves a numerous bodyguard for the express purpose of cowing Paris。
by which = by the very day of capitulation 直到投降那一天
你不要翻译，那我就不翻了~