Damascus is simply a network of spies of that kind--men who attached themselves to the Arab cause when it looked like winning and are now busy transferring their allegiance.
"I think I could name the man who wrote this; I think I know the man who wrote that magnifique.

If I'm right, Yussuf Dakmar will notify the French tonight through their agents in Jerusalem.

The man who wrote that magnifique will know before morning that the letter's missing; and it doesn't matter how careful I may be, it'll be known as soon as I start for Damascus.
"They'll dope out that our obvious course would be to confront Feisul with this letter.

The only way to travel is by train; the roads are rotten--in fact, no auto could get through; they'd tip off the Bedouins, who'd murder everybody.
"So they'll watch the trains and especially Haifa, where everyone going north has to spend the night; and they'll stop at nothing to get the letter back, for two reasons; as long as it's in our hands it can be used to establish proof of the plot against Feisul; once it's back in theirs, they can keep it in their secret dossier to use against Feisul if they ever catch him and bring him to trial.

You remember the Dreyfus case?
"I shall start for Damascus by the early train--probably take an auto as far as Ludd.