Question:  Arthur Machen was a Welsh author and mystic of the 1890s and early 20th century well known for his leading role in creating the legend of the Angels of Mons, a popular legend about a group of angels who supposedly protected members of what army?

full_context: Document 1: A Study in Emerald: "A Study in Emerald" is a short story written by British fantasy and graphic novel author Neil Gaiman. The story is a Sherlock Holmes pastiche transferred to the Cthulhu Mythos universe of horror writer H. P. Lovecraft. It won the 2004 Hugo Award for Best Short Story. The title is a reference to the Sherlock Holmes novel "A Study in Scarlet". "A Study in Emerald" first appeared in the anthology "Shadows Over Baker Street", a collection of stories combining the worlds of Arthur Conan Doyle and H. P. Lovecraft; it has subsequently been available as part of Gaiman's short story collection "Fragile Things," in the collection "New Cthulhu: The Recent Weird", and is available online. The online version takes the form of a Victorian periodical or newspaper, which includes various advertisements that reference characters such as Vlad Tepes, Victor Frankenstein, Spring Heeled Jack, and Dr. Jekyll.
Document 2: Angel (Thomas Halloway): The Angel (Thomas Halloway, often shortened to Tom Halloway) is a fictional character, a superhero appearing in American comic books published by Marvel Comics. Created by artist Paul Gustavson and an unconfirmed writer during the Golden Age of Comic Books, the Angel first appeared in "Marvel Comics" #1 (Oct. 1939), the first publication of Marvel Comics' predecessor, Timely Comics.
Document 3: Abdul Alhazred: Abdul Alhazred is a fictional character created by American horror writer H. P. Lovecraft. He is the so-called "Mad Arab" credited with authoring the fictional book "Kitab al-Azif" (the "Necronomicon"), and as such is an integral part of Cthulhu Mythos lore.
Document 4: Arthur Machen: Arthur Machen ( ; 3 March 1863 – 15 December 1947) was a Welsh author and mystic of the 1890s and early 20th century. He is best known for his influential supernatural, fantasy, and horror fiction. His novella "The Great God Pan" (1890; 1894) has garnered a reputation as a classic of horror (Stephen King has called it "Maybe the best [horror story] in the English language"). He is also well known for his leading role in creating the legend of the Angels of Mons.
Document 5: Munro Ambulance Corps: The Munro Ambulance Corps was started in August 1914 by Hector Munro, who was one of the directors of the Medico-Psychological Clinic in London. The mission of the Corps was to move wounded troops from the battlefield to hospitals in Flanders during World War I. Some of its noteworthy members were British writer May Sinclair, British heiress Lady Dorothie Feilding, and nurses Elsie Knocker and Mairi Chisholm.
Document 6: First Battle of the Marne: The Battle of the Marne (French: Première bataille de la Marne , also known as the Miracle of the Marne, "Le Miracle de la Marne") was a World War I battle fought from 6–10 September 1914. It resulted in an Allied victory against the German armies in the west. The battle was the culmination of the German advance into France and pursuit of the Allied armies which followed the Battle of the Frontiers in August and had reached the eastern outskirts of Paris. A counter-attack by six French armies and the British Expeditionary Force (BEF) along the Marne River forced the Imperial German Army to retreat north-west, leading to the First Battle of the Aisne and the Race to the Sea. The battle was a victory for the Allies but led to four years of trench warfare stalemate on the Western Front.
Document 7: Angels of Mons: The Angels of Mons is a popular legend about a group of angels who supposedly protected members of the British Army in the Battle of Mons at the outset of the First World War.
Document 8: The Heroine of Mons: The Heroine of Mons is a 1914 British silent war film directed by Wilfred Noy (the maternal uncle of Leslie Howard) and starring Dorothy Bellew, Leslie Howard and Bert Wynne. The film marked the screen debut of Howard, who went on to be leading star of British and Hollywood cinema. The film was made during the opening weeks of the First World War, and refers to the Battle of Mons.
Document 9: Weird fiction: Weird fiction is a subgenre of speculative fiction originating in the late 19th and early 20th centuries. It can be said to encompass the ghost story and other tales of the macabre. Weird fiction is distinguished from horror and fantasy in its blending of supernatural, mythical, and even scientific tropes. British authors who have embraced this style have often published their work in mainstream literary magazines even after American pulp magazines became popular. Popular weird fiction writers included Edgar Allan Poe, William Hope Hodgson, H. P. Lovecraft, Lord Dunsany, Arthur Machen, M. R. James, and Clark Ashton Smith.
Document 10: Sheridan Le Fanu: Joseph Thomas Sheridan Le Fanu ( ; 28 August 1814 – 7 February 1873) was an Irish writer of Gothic tales and mystery novels. He was a leading ghost story writer of the nineteenth century and was central to the development of the genre in the Victorian era. M. R. James described Le Fanu as "absolutely in the first rank as a writer of ghost stories". Three of his best-known works are "Uncle Silas", "Carmilla" and "The House by the Churchyard".
Document 11: Parliamentary War Memorial: The Parliamentary War Memorial, also known as the Recording Angel Memorial, is a stone sculpture in Westminster Hall, unveiled in 1922, which commemorates the members of both Houses of Parliament of the United Kingdom who died in the First World War. It names 22 members of the House of Commons, 20 members of the House of Lords, and 9 senior members of staff, together with another 94 sons of members and officers of the House of Commons, who lost their lives in the war. (Sons of peers and of officers of the House of Lords are commemorated on the wooden panels of the House of Lords War Memorial in the Royal Gallery.) Above the memorial is a large stained glass window which commemorates members and staff of both Houses who died in the Second World War.
Document 12: Angelus: The Angelus ( ; Latin for "angel") is a Catholic devotion commemorating the Incarnation. As with many Catholic prayers, the name "Angelus" is derived from its incipit: "Angelus Domini nuntiavit Mariæ" ("... the Angel of the Lord declared unto Mary ...") and is practised by reciting as versicle and response three Biblical verses narrating the mystery; alternating with the prayer "Hail Mary". The Angelus exemplifies a species of prayers called the "prayer of the devotee".
Document 13: The Great God Pan: The Great God Pan is a novella by Welsh writer Arthur Machen. A version of the story was published in the magazine "The Whirlwind" in 1890, and Machen revised and extended it for its book publication (together with another story, "The Inmost Light") in 1894. On publication it was widely denounced by the press as degenerate and horrific because of its decadent style and sexual content, although it has since garnered a reputation as a classic of horror. Machen’s story was only one of many at the time to focus on the Greek God Pan as a useful symbol for the power of nature and paganism. The title was possibly inspired by the poem "A Musical Instrument" published in 1862 by Elizabeth Barrett Browning, in which the first line of every stanza ends "... the great god Pan."
Document 14: Fallen Angels (Black Veil Brides song): "Fallen Angels" is the third single by the American rock band, Black Veil Brides, and the first single from their second album "Set the World on Fire". The song is inspired by the biblical story told in the Book of Revelation, Chapter 12, in which Satan and one-third of God's angels rebelled against God, starting a war in Heaven and were therefore cast out of Heaven to the earth, becoming "fallen angels." The story was introduced to Andy Biersack and the Black Veil Brides by their band artist Richard Villa:
Document 15: H. P. Lovecraft: Howard Phillips Lovecraft ( ; August 20, 1890 – March 15, 1937) was an American author who achieved posthumous fame through his influential works of horror fiction. He was virtually unknown and published only in pulp magazines before he died in poverty, but he is now regarded as one of the most significant 20th-century authors in his genre. Lovecraft was born in Providence, Rhode Island, where he spent most of his life. Among his most celebrated tales are "The Call of Cthulhu" and "The Shadow over Innsmouth", both canonical to the Cthulhu Mythos. Lovecraft was never able to support himself from earnings as author and editor. He saw commercial success increasingly elude him in this latter period, partly because he lacked the confidence and drive to promote himself. He subsisted in progressively strained circumstances in his last years; an inheritance was completely spent by the time that he died at age 46.
ground_truth: the British Army