“One may as well begin with Socrates.” So opens Nikhil Krishnan’s “A Terribly Serious Adventure: Philosophy and War at Oxford, 1900-1960.” Observe how that sentence conveys a certain offhand jauntiness, yet subtly reveals a distinctly learned author: Krishnan, who teaches philosophy at Cambridge, implies he could have chosen any number of other philosophers instead of Socrates. At the same time, his decision to create this particular sentence indicates a writer confident enough, and well read enough, to echo one the most famous first lines in 20th-century British fiction: “One may as well begin with Helen’s letters to her sister,” the opening of E.M. Forster’s “Howards End.”