Greenmantle is the second of five novels by John Buchan featuring the character Richard Hannay. It was first published in 1916 by Hodder & Stoughton, London. It is one of two Hannay novels set during the First World War, the other being Mr. Standfast (1919); Hannay's first and best-known adventure, The Thirty-Nine Steps (1915), is set in the period immediately preceding the war.

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.

The book was very popular when published, and was read by Robert Baden-Powell and by the Russian imperial family as they awaited the outcome of the revolution in 1917.

"A Mission Is Proposed", the first chapter of Greenmantle, was chosen by Graham Greene for his 1957 anthology The Spy's Bedside Book.

The book has been adapted for broadcast on BBC Radio 4. It was broadcast on BBC Radio4 Extra in two episodes on 27 and 28 August 2013, and again on 30 April and 1 May 2015, with David Robb as Richard Hannay and James Fleet as Sandy Arbuthnot, forced to be 'Greenmantle': . (wikipedia.org)