Isbell was promoted to Captain in 1942 while in command of the Naval Air Station, Sitka, Alaska.
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Toynbee was the uncle, via his brother Harry Valpy Toynbee, of universal historian Arnold Joseph Toynbee (1889–1975); with whom he is often confused.
He was a prolific historical author, best known for his abridgement (still in print) of Arnold J. Toynbee's A Study of History.
After graduation, Isbell was drafted into the U.S. Army, and stationed at the Aberdeen Proving Ground.
Born into a long-established Bedfordshire family, he was educated at Winchester College, a contemporary of Arnold J. Toynbee and R.M.Y. Gleadowe, and at New College, Oxford, which he left with a First in Classical Moderations in 1911.