Greenmantle (Version 2)

Greenmantle is the thrilling second installment in John Buchans five-book Richard Hannay series, first published in 1916 by Hodder & Stoughton, London. Set against the backdrop of the First World War, this gripping tale follows Hannay as he investigates unsettling rumors of an uprising in the Muslim world. Embarking on a dangerous journey through enemy territory, he seeks to reunite with his friend Sandy in Constantinople. Together, they must navigate treacherous plots and thwart the Germans’ sinister plans to manipulate religion in their quest for victory, culminating in the dramatic battle of Erzurum. - Summary by Wikipedia