It happened to me to DM recently a couple of adventures both steeped in a real-world historical context: the first was a module for Delta Green set in 2009 Afghanistan, while the second was an adventure for Lex Arcana set in Armenia around 428 AD.
The preparation of both modules led me to delve to a certain degree into the social and historical context of their respective settings. Being interested in the reconstruction of real situations, I enjoyed learning more about those two realities and coming up with details that could better convey the feeling of the time.
However, while I gladly spent hours consulting books and Wikipedia entries, a couple of questions naturally arose in my mind.
To what degree of accuracy should I reconstruct the historic setting? Given that the reconstruction will always be, to a certain point, faulty, it is still reasonable to wonder up to what degree pursuing accuracy is a worthy aim. The risk is, of course, to bury the players in details and to transform a session into a history class.
How to balance the amount of detail depends on many factors differing at each individual table, starting with the interest and the participation of the players in the effort of re-creating a world true to a certain historical reality. Obviously, if this is not among the priorities, any insisting on historical accuracy may feel like an imposition.
Still, if players are open to trying to reimagine situations faithful to a historical reality, it may be necessary to balance the time spent by the GM in historic description with the actual playing time. Here, it seems to me, the GM faces a challenge analogous to the one every writer has to consider: how to depict a possibly unusual or distant situation in a limited amount of space and words.
In my experience some descriptions are more effective than others. Dialogic descriptions which invite questions are particularly useful, simply because they transform a potential monologue into a participated discussion. Similarly, PC-related descriptions, which connect and rely on the background of the players, are also helpful because they catch two birds with one stone: characterizing the setting and providing hooks for role-playing. I also favour worldview descriptions, that is the narration of both small and big aspects that may convey ideas and views unique to the setting being played.
In all cases, a certain priority should be given to descriptions that can summon vivid images and feelings. The feeling of impassability of the range of mountains is more important than their actual height; the feeling of lineage and tradition of a royal house is more important the actual number of monarchs. Historical details may be appreciated by some players, but it should not be forgotten that normally we are at the table for a game, not for a scholarly reconstruction.
Although, I would be myself quite strict in recreating historical situations, it is probably best if historic constraints are used in the service of the game, and not vice versa. As in historical fiction, historical accuracy may sometimes be sacrificed for the sake of narrative.
Still, in the name of accuracy, I enjoy telling my players at the end of the session what elements were fiction, and what was actual history. Often the revelation of historical facts and truths is a rewarding source of awe and learning.
What is the aim of adding historic background? To weigh the importance of accurately reconstructing a historical background we need to understand what the role is of such a reconstruction.
It is possible that some of the players may enjoy the plain possibility of learning something about another time. In such cases, describing real-world locations or events might be a positive feature in itself. However, let us make here the assumption that the game is not expected to be educational in such a factual way.
An important contribution consists, I think, in the recreation of a unique and compelling context, separated and distinct from our everyday world or from other familiar settings.
A well-reconstructed historical background may offer the possibility to truly immerse ourselves in different worlds and role-play characters with new and diverse perspectives. It offers a precious opportunity of imagining a new world, which, at the same time, is, or was, the world lived and experienced by other human beings. This could allow space for experiencing new worldviews or trying out different societal norms.
More than other media types, a good RPG session may offer the possibility of living in other realistic worlds, and history may offer a deep and solid framework upon which to build these settings, as well as plethora of material about which to think and ponder.