hidden historical background; behind-the-scenes history
Refers to the lesser-known, often unofficial or concealed aspects of historical events, as opposed to mainstream narratives.
This book describes the hidden history of the war in detail.
If you're interested in behind-the-scenes history, I recommend this documentary.
秘史 means 'secret history' and emphasizes deliberately concealed or confidential information, whereas 裏面史 can include simply overlooked or unofficial aspects.
Compound of 裏面 (behind the scenes, reverse side) and 史 (history). The exact origin is uncertain, but it follows a common pattern for formal historical terms.