John Barnes, aka the Sphinx, is an agent with American Intelligence.
Formally, he is nothing of the kind. There are no agents, apparently. The activities of Barnes and his compatriots take place just a couple years after the U.S. State Department suspended all intelligence work it had been involved in (Secretary Henry Stimson's line "Gentlemen do not read each other's mail" comes to mind).
So it will fall on the shoulders of independent men (and women) to take over the very dangerous and unheralded work of spying on other nations and organizations to keep America from being totally in the dark. Barnes is one such man.
In the first adventure, at the very beginning, an Ambassador, sending him on what appears to be his first mission, warns, "It's a new deal all around, Barnes, at home and abroad. We're fighting these Europeans with their own weapons for a change. Remember, you've no earthly connection with Washington! You and the other chaps in the game are taking your chances - long chances, Barnes. You're devoting your time, money, and lives to a cause. You'll have no reward except the satisfaction of serving your country. If you're caught - goodnight!"
This self-reliance is echoed in the second adventure when an opposition leader rather unflattering comments, "Certain Americans like Barnes are working for their government. Idealistic fools, who place themselves and their brains and money at the service of their country; they have no standing, they have no acknowledged connection with Washington. They are free-lances who prate of bringing a new deal into diplomacy, of fighting us here in Europe with our own weapons. Barnes is one. He pretends to be a fool, but is smart enough."
In order to accomplish all that was needed to be done in the ever-changing landscape of post-WWI Europe (and Asia), people already experienced with the area are chosen. "Barnes knew Europe. He had lived abroad for several years. He had been in business in Paris. Level-eyed, quiet, seldom losing his head, Barnes was no Herculean figure; that was how he got results. Few people suspected his capabilities."
As his time progresses, his cleverness and his intellect will bring together a small but impressive network of agents. Also growing is the unofficial admiration (and oddly aggravation) by the high-level diplomats who make use of the intel he gathers. And, unfortunately, the number of people who would love to do away with him for the successes he has achieved.
Barnes got his nickname of the Sphinx at the end of the first recorded adventure when Marie Nicholas (read the tale to learn more of her and how she became one of Barnes' best operatives) chided him with, "oh, how you tricked us all! You, with your innocence, your naive cantrap, your pretended childishness; a Sphinx, that's what you are! A rascal!" He was rather taken with the sobriquet. [Oh, cantrap means "a charm, an incantation, adroit mischief" - you're welcome!]