From 1911 Encyclopedia Britannica, we find “forlorn hope” as defined as, “FORLORN HOPE (through Dutch verloren hoop, from Ger. verlorene Haufe=”lost troop”; Haufe, “heap,” being equivalent in the 17th century to “body of troops”; the French equivalent is enfants perdu), a military term (sometimes shortened to “forlorn”), used in the 16th and 17th centuries for a body of troops thrown out in front of the line of battle to engage the hostile line, somewhat after the fashion of skirmishers, though they were always solid closed bodies. These troops ran great risks, because they were often trapped between the two lines of battle as the latter closed upon one another, and fired upon or ridden down by their friends; further, their mission was to facilitate the attacks of their own main body by striking the first blow against or meeting the first shock of the fresh and unshaken enemy. In the following century (18th), when lines of masses were no longer employed, a thin line of skirmishers alone preceded the three-deep line of battle, but the term “forlorn hope” continued to be used for picked bodies of men entrusted with dangerous tasks, and in particular for the storming party at the assault of a fortress. In this last sense “forlorn hope” is often used at the present time. The misunderstanding of the word “hope” has led to various application of “forlorn hope,” such as an enterprise offering little hope of success, or, further still from the original meaning, to the faint or desperate hope of such success.”

A forlorn hope is a band of soldiers or other combatants chosen to take the leading part in a military operation, such as an assault on a defended position, where the risk of casualties is high.

Etymology
The term comes from the Dutch verloren hoop, literally “lost troop”. The Dutch word “hoop” can mean “hope” but is in this context etymologically equivalent to the English word “heap”. The term was used in military contexts to denote a troop formation. The Dutch word hoop (in its sense of heap in English) is not cognate with English hope: this is an example of false folk etymology. The mistranslation of “verloren hoop” as “forlorn hope” is “a quaint misunderstanding” using the nearest-sounding English words. This false etymology is further entrenched by the fact that in Dutch the word hoop is a homograph meaning “hope” as well as “heap,’ though the two senses have different etymologies.

William Barnes Wollen – ‘Battles of the Nineteenth Century’ by Archibald Forbes, G.A. Henty and Arthur Griffiths
Illustration of Colin Campbell leading the ‘forlorn hope’ at the Siege of San Sebastián, 1813 – Public Domain http://en.wikipedia.org
/wiki/Forlorn_hope#
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Forlorn_hope.jpg

History
In the days of muzzle-loading muskets, the term was most frequently used to refer to the first wave of soldiers attacking a breach in defences during a siege. It was likely that most members of the forlorn hope would be killed or wounded. The intention was that some would survive long enough to seize a foothold that could be reinforced, or at least that a second wave with better prospects could be sent in while the defenders were reloading or engaged in mopping up the remnants of the first wave.

A forlorn hope may be composed of volunteers and led by a junior officer with hopes of personal advancement. If the volunteers survived, and performed courageously, they would be expected to benefit in the form of promotions, cash gifts and adding glory to their name. The commanding officer himself was almost guaranteed both a promotion and a long-term boost to his career prospects. As a result, despite the risks, there was often competition for the opportunity to lead the assault. The French equivalent of the forlorn hope, called Les Enfants Perdus or The Lost Children, were all guaranteed promotion to officer rank should they survive, with the effect that both enlisted men and officers joined the dangerous mission as an opportunity to raise themselves in the army.

By extension, the term forlorn hope became used for any body of troops placed in a hazardous position, e.g., an exposed outpost, or the defenders of an outwork in advance of the main defensive position. This usage was especially common in accounts of the English Civil War, as well as in the British Army in the Peninsular War of 1808–1814.

Starring: Henry Arousell, Thomas Boqvist, Marko Kattilakoski, Johan Klint, Bengt Westin, Dennis Åhs
Genre: 40 min short, 17th century war/horror. Honorable mention Best Period Film H. P. Lovecraft Film Festival 2009, Portland Oregon USA.
Directed By: Johan Karlsson
http://www.imdb.com/title/tt1813317/

There is a recent 40 minute short film that portrays “Forlorn Hope,” which I found interesting. A group of scattered soldiers that are lost after the Battle of Breitenfelt meets a man who guides them on a forlorn mission. Soon death and mistrust are spreading within the group and a a horrifying showdown becomes inevitable.

The movie represents an occurrence where one of Nyarlathotep forms makes its presence known in Germany 1631. A group of scattered soldiers that are lost after the battle of Breitenfelt meets a man who guides them on a forlorn mission. Soon death and mistrust are spreading within the group and  a horrifying showdown becomes inevitable. (imdb)

Parts of this piece are furnished by Wikipedia. Other references are so noted.