To make good use of history in an RPG you need to decide how you will approach history and how to address some of the problems it creates at the table.
HOW DOES HISTORY WORK: Everyone has an opinion about history. Some subscribe to grand theories while others focus on the big movers and shakers. Is history set on a firm path? Is it a build-up of micro-level events? Are there key turning points in history or are those an illusion because history is directed by massive forces that are greater than individual men and battles? How you answer these sorts of questions will directly impact the game and how much infuence the PCs have over future outcomes. And fundamentally that is the key concern: can the PCs actions change the future?
CAN THE PCs CHANGE THE FUTURE: This is entirely up to you (and builds on how you answer the previous question). Keep in mind that much of this is situationally depenant. Even if you decide that bit players like the PCs can change hisory's course, they still need to be in the right place and time to do so. They also need access to the means. It can be great fun to out a little power in the player's court and give them a hand in shaping history. As a general rule though their ability to do so should be limited by the amount of power they wield or have access to. The Emperor can change the course of history with a single word, but a single soldier in the Roman army, even if he wants to, will have a hard time changing things. Afterall most people have enough trouble mastering theor own fate let alone the fate of the world. And to truly manipulate history, you need foreknowledge of it, which the PCs don't normally have. This raises a big concern in historical RPGs: metagaming.
METAGAMING: This is a huge problem in some historical campaigns. The players know the future, but their PCs do not. So how do you deal with this? An easy way is to change the future enough that the
players' real world knowledge has zero impact. Another approach is to step in at times and prevent the players from metagaming. Simplay say "your character doesn't know Cassius is the future assassin of Caligula so he has no reason to stab him in the back. The problem with the later is it can impinge on player freedom, however if the player is genuinely abusing real world knowledge, most people at the table shouldn't object. Of course even if the players have the power to change history (let's say they stab Cassius before he is in a position to murder Caligula) that can be a good thing because it launches you into a sea of unknowns.
THE SEA OF HISTORY: Anytime the players (directly or indirectly) alter the course of history, things change. The more they change, the less future campaign events will match our understanding of histoy. To use the above example, if the PCs kill Cassius, then everything after that is up in the air (at least everything related to the the imperial family). At least the players knew where and when the historical conspiracy against the emperor was, but if they take out its ringleader, that doesn't mean there won't be more. Most likely another conspiracy emerges and could succeed.
But everything that followed form the original conspiracy needs to be re-examined. We are no longer sure if Claudius succeeds Caligula for example. And this raises all kinds of interesting possibilities. Who will be the next emperor? Will there even be another emperor? Perhaps the next conspiracy restores the Republic or divides the empire into multiple parts. Maybe civil war breaks out again. Who knows. What about a Britannia? If Claudius isn't the next emperor, we can't be sure of its conquest (perhaps the next emperor doesn't need further military victories to secure his standing).
(Continued in Part Four)