Every narrative begins in media res. Perhaps temporally. Perhaps along some other dimension. But all narratives begin in some middle of "things".
It is conventional in the historical narrative to begin with an episode, some specific event, clearly delimitated and distinct. An iconoclastic monk plunges his dagger into the back of a powerful king; a frightened Telemachus faces the assembly and fights in that typically Greek way, with lengthy, legalistic debate; or, in a more recent example, the Prussian government takes the first step in creating a new forestry policy in 1765. In media res, the form it sometimes takes. A single, striking moment is given to the audience. It needs to be sufficiently small and self-contained to be readily apprehended. Yet, it must be suggestive enough to introduce the broad themes that will follow. Whatever the final shape of the complete work, whatever the relation of the beginning to the whole, the narrative is built around this introductory core.
After the initial, violent moment, the narrative is quickly expanded, both forward and backward. The King's guards and entourage respond to the monk-turned-assassin. After the initial shock of revenge, the kingdom grieves. Thus, we learn about the past, that the old King was a good King (at least relative to what will follow). Lines of political tension begin to emerge, radiating outward through court and countryside, radiating from that first moment of the narrative. Thus we learn about the future, the dark clouds that loom just over the horizon, the chaos that is to come. This being the Middle Ages, those political tensions are inextricably mixed with the religious. Thus we learn yet more about the past, for the religious tensions explain and illuminate the monk's actions, and vice versa. We began with two men and the swift movement of a single dagger. Yet now, just a few words later, we have come to the politics of kingdoms and the theology of a continent.
Every narrative begins in media res. We must begin somewhere, with some initial scene, and then proceed from that place. Perhaps it will be somewhere in the chronological middle, as in the traditional definition of the term. Such is a good way of organizing ideas, for it reflects the way--the only way--we who are forever embedded in the stream of consciousness can experience them. Yet even if the organization is thematic, the basic structure of the narrative remains much the same. Instead of a violent moment, perhaps the opening salvo is a quote. Still, the function of the quote is much the same as the momentous event. It induces some strong visceral reaction because it is surprising or shocking or tantalizing, yet the quote itself and its exegesis are on a small enough scale that the initial dramatic arc is quick and intelligible. One can and should come to larger points in the course of the narrative, but without context, or more precisely, with only the context possessed by the general intelligent reader, such broad points are initially out of reach. Better to give the reader an interesting morsel, a glimpse of the broader intellectual vistas to come, and then retreat a bit, repeating something said earlier or otherwise telling the reader something they already mostly know, as a chaser for the preceding suggestion.