One of the aspects of world design that is often overlooked it’s history, too often it is seen as either unimportant or something that can be made up on the fly. I disagree with both those assessments. If one looks at the various published campaign settings, they all have their histories detailed to some degree, which adds to their flavour and also allows the DM to create logical and consistent adventures and campaigns. It also allows the players an opportunity to role-play better as they are not working blind so to speak, they have a deeper knowledge of the world they are adventuring
However, too detailed a history can also be something of a straitjacket for the DM, dictating story lines rather than presenting possibilities. Therefore making one’s history is something of a juggling act. What I have always done is to outline the general course of the history without detailing it too much, this allows me to insert things into the timeline as I need them for whatever campaign I am running at the time.
There are two methods I have used, the first is simply an outline of the history, such as this one I made for my Zaidan-Tai PBEM.This approach is best for smaller settings, or for the immediate campaign area. For larger settings, or for the wider world beyond the immediate campaign area I generally make a series of maps, sort of an historical atlas, tracking the the rise and fall of the various nations.
Given the scope of the Tòlanar setting, I have done the latter, making maps starting at an arbitrary date, which you can see here.
One thing you will see in either approach is that in both cases they are simply outlines, this is deliberate. First of all writing a detailed history of an entire world or continent is far too great a task, and second, doing so limits one’s options when creating adventures and actually playing. By leaving things vague in your overall history, you are free to fill in the blanks as needed for the game, filling out more details as the scale gets smaller. It also allows the players to have input as well, as they, knowing the general outline, can fill in details themselves, thereby contributing to the world, making it richer and also taking a lot of work out of the DM’s pile. It also helps the players to feel more invested in the setting as they have literally helped build it.