I recently saw a framed original of this poster in a private residence.
It advertises a fundraiser, held 100 years ago today at the no doubt tony address of 26 Regent Street, London, for The Mesopotamia Comforts Fund for British troops. The money raised went toward the support of British troops in Mesopotamia.
Many people's reaction today to the names of some of the event's headliners might understandably be, "Sounds like something from Downton Abbey!".... The Countess Dowager of Carnarvon, The Countess of Minto, and Lady Mildred Allsopp.*
During World War One (a.k.a. the Great War, the First World War, often abbreviated WWI), British General Sir Frederick Maude led a series of successful operations in the Mideast that culminated with the capture of Baghdad on March 11, 1917. General Maude died in November 1917 in Mesopotamia from cholera while leading the next phases of successful British operations in the region.
More details about the poster are on the website of the Imperial War Museum (IWM).
*The Carnarvon's estate, Highclere Castle in Hampshire, England, was used as Downton Abbey. The Dowager Countness mentioned in the poster would have been Elizabeth Catherine Howard (1856-1929), Fourth Countess of Carnarvon, who would have become the Dowager Countess upon the death of her husband Henry, Colonel Secretary and Lord Lieutenant of Ireland, in 1890. (See the current Lady Carnarvon's website at LadyCarnarvon.com. As my mother might say, "It's a hoot.")