When Americans look back on their great Civil War from 1861 to 1865, and they’re doing that a lot right now, they tend to see it as exclusively an internal affair. Hundreds of thousands died in horrific battles. The slaves were freed. The Union saved. And what was going on across the Atlantic was hardly relevant. But as Amanda Foreman makes abundantly clear in her massive, magisterial history A World on Fire: Britain’s Crucial Role in the American Civil War, nothing could be further from the truth. There’s a lot here worth thinking about in terms of today’s geopolitics. When should a great power get involved in a distant conflict? When should it keep its distance? London’s dilemmas then are not so different from Washington’s today.