For millions of years, the river Ebro has sloshed south from Spain’s jagged Cantabrian Mountains, carving out a broad valley that is now home to one of the country’s most fertile wine regions. Between its sprawling vineyards, the landscape rises steeply to hilltop medieval towns. Laguardia is the best known, on account of its high walls, cobblestones, and cavernous wine cellars. But the town’s rustic grandeur conceals a deep history of violence. More than 2,000 years ago, Celtic tribes fought a decades-long series of wars in this region, part of a brutal last stand against the invading Romans—and for Laguardia, even those conflicts were of relatively recent vintage. Some years ago, just outside the town walls, workmen at a construction site were operating a bulldozer when one of them spotted bones sticking up through the disturbed earth. Archaeologists were dispatched to the scene. Careful brushwork revealed not one human skeleton but 90, along with pieces of more than 200 others, all dated to a little more than 5,000 years ago. A new analysis of the …