It seems that this idea was older than I thought.
This article, from the Brooklyn Daily Eagle of October 14, 1877, describes fearful - and literal - Flag of Islam that would signal an all-out war of Muslims against Christians. Supposedly when this flag, housed in a mosque in Constantinople, would be unfurled, it would signal every single Muslim to fight to achieve paradise.
Note the fear that the author shows of the possibility that huge crowds of Mohammedans could be unleashed at a moment's notice to kill every Christian.
Here are some highlights:
Since the commencement of war between Russia and Turkey, the world has several times been startled by the announcement that the "Flag of the prophet" was about to be unfurled in the streets of Stanbul. Such an event, if it should happen (which may Heaven avert), would proclaim a crusade in which all true Musslemans would be bound to take an active part and to fight against Christianity in every part of the world. They may be in India, Arabia, Egypt or wherever else their scattered race has found a home; the raising of the green standard is a call in which none may disobey without, as the Koran lays it down, sacrificing all his hopes of Paradise.
This fearful appeal to all the worst passions of the Eastern races hangs like a menace over the Mohammedan world; and if the word was once uttered and the dreadful flag unfurled, there is no telling to what sanguinary excesses it might lead an enthusiastic people. If may be of interest to our readers if, under these circumstances, we endeavour to make them acquainted with the origin and history of a banner that has not seen the light of day since the Empress Catherine of Russia attempted to reinstate Christianity in the city of the Sultans, and which once unfurled would set the world ablaze.
The prophet himself predicted that one day, when his followers should number 100,000,000- which they do now with 20,000,000 more added to it- his flag should fly against the advancing power of the northern races; and the Koran Mohammedan Bible says that when its silken folds are flung forth "the earth will shake, the mountain melt into dust, the seas blaze up in fire, and the childrens hair grows white with anguish." This language is of course metaphorical; but it is easy to conceive, by the light of very recent history, that some such catastrophe might take place, as the displaying of this terrible symbol would raise a frenzy of fanaticism in the breast of the Mohammedan race all over the globe.
When once unfurled, it summons all Islam by an adjuration from the Koran that the sword is the solitary emblrm and instrument of faith, independence and patriotism; that armies, not priests, make converts and that sharpened steel is the "true key to heaven and hell"
Upon that fearful ensign are inserted the words which are supposed to have been written at Mecca itself- namely, "all who draw it (the sword) will be rewarded with temporal advantages, every drop shed of their blood, every peril and hardship endured by them, will be registered on high as being more meritorious than either fasting or prayer. If they fall in battle, their sins will be at once blotted out, and they will be transported to Paradise, there to revel in eternal pleasure in the arms of black-eyed hours. But for the first heaven are reserved for the faithful who die within sight of the Green Flag of the Prophet" Then follow the terrible and all-significant words, the fearful war-cry against God and man, "Then may no man give or expect mercy."
This is the outburst of barbarism with which the world is threatened in this year of grace, 1877; and the reader cannot do otherwise than mark the cunning nature of the portentous works inscribed on the prophets banner. What would not most men do, civilized or savage, for "temporal advantages"? While to the eastern people fasting and praying, are looked upon as of so meritorious a nature, than to find something else which, in the eyes of Allah, would be desideratum which none would fail to grasp by any means whatever, if it came within its reach. But Mohammed's wonderful knowledge of human nature, is shown in his picture of Paradise as prepared for the faithful who fall in battle while his declaration that the highest heaven in this so called Paradise will be reserved for those who die within sight of the Green Flag, is a masterpiece of devilish policy unequalled in the annals of mankind.
It scarcely needed the fearful words which follow, to add emphasis to this dreadful appeal to the passions of a semi-barbarous race. Another motto on this sacred flag is not without significance at the present time; "The gates of Paradise are under the shade of swords;" and this alone would, if the flag were unfurled in the Holy Mosque of Constaninople, give to the Turk a moral power over his subordinates the effect of which it would be vain to calculate. Civilized though he partially is , he still firmly believes in the old doctine of kismet or fatality, and in angels fighting on his behalf; not less implicitly than did his ancestors at the Battle of Bender, where this formidable green standard was first unfurled...