And in more ways than one.
From 1915 onwards, stories surrounding the retreat at Mons became widespread in Christian and Theosophical magazines as well as by word of mouth. Various "eyewitness" accounts reported that the British troops received supernatural protection during their retreat. The accounts ranged from a "strange mist" that hid soldiers from the Germans, phantom horsemen led by St. George himself, or even a host of angels. While similar stories had been told in medieval battles, this was the first case of its kind in modern warfare. While there were certainly reports of mass hallucinations taking place during that period (including the "Miracle of the Sun" at Fatima in 1917), the Mons event doesn't appear to be one of them. All of the accounts of sightings were second- or third-hand involving either friends of soldiers or nurses who tended the soldiers afterward.
Despite the flimsy nature of the actual evidence for the Angels of Mons sightings, the British spiritualist magazine Spiritualist published details of the "miracle" in a 1915 issue and the controversy was on. The story of the miraculous events at the Battle of Mons was repeated in religious sermons across the U.K. and in newspapers around the world. The war was already unpopular as the British people realized just how long and vicious it would be. The notion that God was on the Allied side and ready to strike down the evil Germans helped boost morale. While WWI progressed with horrendous casualties and no evidence of further divine intervention, the story of the Angels of Mons became one of the most often-repeated tales of the war (and one of the first true urban legends of modern times).
There is still controversy over the original source of the rumours but the most likely explanation involves a short story by Arthur Machen that was printed in The Evening News on September 29, 1914. Titled "The Bowmen", Machen's story was written from a first-person perspective and had St. George appear at the Mons battle site with an army of phantom archers to save the retreating British troops. While Machen was employed as a journalist at the time, he was also an accomplished fantasy and horror writer who often wrote about supernatural themes. Since Machen was one of the newspaper's regular correspondents and the story wasn't clearly identified as fiction, there was some confusion over whether his story was meant to be taken as fact. Shortly after the story's publication, Machen received numerous requests from parishes across Britain to republish the story and he was also asked for the source of his story. Machen was quick to insist that his story was completely fictional but various retellings changed the story, and its fictional origins, almost beyond recognition (somewhere along the way, his phantom bowmen were transformed into angels).
Although Machen continued to emphasize that his story was pure fiction, spiritualists claimed that he had based his story on rumours that had already been spread by British soldiers. The story became so popular that it became downright "unpatriotic" to dispute it (which placed skeptics in an awkward position). One possible source of these claims came from letters that were apparently written by Brigadier-General John Charteris in 1914-1915. In the letters, which were published in Charteris' memoirs in 1931, he noted that "the story of the Angels of Mons [is] going strong through the 2nd Corps". While the letter appears to predate Machen's story, there is some controversy over whether the dates were falsified considering that the letters themselves can't be found in Charteris' actual archives. No other first-hand testimony was ever recorded. While Arthur Machen played up the fictional nature of his story, his book The Bowmen and other Legends of the War published in 1915 became a runaway success selling 3000 copies in London on the first day it came out. The fact that the book's introduction carefully explained how his original story became a legend did nothing to stop the rumours.
So why did the rumours around the Angels of Mons persist? Part of the reason could be that World War I represented one of the first modern conflicts where propaganda played a major role in boosting morale for Allied troops and civilians alike. Along with the Angel rumours, there were stories of German atrocities such as crucifying Allied soldiers, killing and mutilating Belgian women and children, and executing Allied nurses. Since the war was becoming increasingly unpopular as the death toll rose, military authorities often depended on disinformation campaigns to drum up popular support (Brigadier-General Charteris was part of British Army Intelligence). Whether or not the military actively promoted the angel story, the notion that God had provided heavenly help for the Allies played well to churchgoers in Britain (even though there was no divine support in later battles). Long after World War II ended, believers and skeptics alike still searched for first-hand testimony of angel sightings without success.
Though largely forgotten today, the Angels of Mons rumour and its persistence over time is a powerful example of how urban legends can spread. As new legends arise, skeptics are continually reminded of how hard it is to dissuade people in believing in them, especially when they're being told what they want to hear.