"Greenmantle" is a spy novel written by John Buchan and published in 1916. Set during World War One, "Greenmantle" is the second of the 5-book series of the Richard Hannay novels which immediately follow "The thirty-nine steps", set a few months before the beginning of the war. In "Greenmantle", Richard Hannay will fight an incredible German plot to win the war from the East, from the Orient, through the stirring of religious sentiment.

It is November, 1915. Major-General Sir Richard Hannay is called in to investigate rumours of an uprising in the Muslim world, and undertakes a perilous journey through enemy territory to meet his friend Sandy in Constantinople. Once there, he and his friends must thwart the Germans' plans to use religion to help them win the war, climaxing at the battle of Erzurum.