A World on Fire: Britain’s Crucial Role in the American Civil War By Amanda Foreman (Random House, 958 pp., $35) The world’s biggest superpower has a problem. The citizens of a nation overseas have risen up against their tyrannical rulers, determined to claim liberty even if it takes a civil war. As the most powerful global advocate of freedom, the superpower has to admire the rebels’ cause. Should it help them? Humanitarians argue that intervention can prevent hundreds of thousands of civilians from suffering hideous state-sponsored subjugation.