Discussed in this essay:

Spain in Our Hearts: Americans in the Spanish Civil War, 1936–1939, by Adam Hochschild. Houghton Mifflin Harcourt. 464 pages. $30.

Some eighty years ago, several thousand young Americans went to Spain to fight — and, many of them, to die — in the service of a country that was not their own. Nothing like that had happened on such a scale before, and nothing like it has happened since. They went despite the active opposition of their own government, which would treat them, upon their return and for many years thereafter, as politically suspect. They left jobs,…