How ought we to view history? Are we the apotheosis of some sort of inexorable, preordained script? Or is history contingent, accidental and therefore exciting and relevant? Was there once a multiverse of potential outcomes? It’s all too easy to see the Civil War through the wrong end of the telescope — distant, small, and as inevitable as any star in the firmament. But Richard Slotkin, in “The Long Road to Antietam,” flips that telescope around and allows us to view history as it was seen by the participants. No outcome was certain, motives were mixed, and events were a …