I've come to feel that at least half the questions on this site, as different as they are on the surface, all have the same answer: Fully work out all the details of your backstory, richly imagine it, make sure it makes sense, but then only include in the actual book the pieces that are necessary to the narrative as it unfolds. In this case, however, as I understand it, there are two additional wrinkles. You want enough of the backstory that the reader can gain an apprehension of it, but you don't want the characters to also figure it out.
I would advise caution --it would be easy to deform your story to the point of unreadability in pursuit of a goal like this --I've seen it happen to books. Your best bet, I think, is to make sure that a) the backstory is relevant to the current events, so that it shows up organically, without being shoehorned in and b) that the characters don't put the pieces together because they are interested in other things --maybe they have more pressing concerns, maybe they aren't history buffs, maybe they all know the backstory already and just take it for granted (probably the choice with the best chances of success).
If the characters are pursuing the backstory, if all the hints are in the book, and if the characters still fail to figure it out, that's just going to feel like a cheat. Alternately, you might have one character figure it out, and just not share the answer explicitly with the reader, leaving it as a soluble puzzle. Even better advice, however, might be to do as Tolkien did: Leave the backstory out of the book (except as needed) and just continue to mine it for additional books later.