Going back to a comment I had made in the question on What were the types of sieges something I had seen often referred to in the Bernard Cornwell Sharpe series was the Forlorn Hope. Basically these were the soldiers who were first through a breach in the wall, a sort of suicide attack on a well defended position the typical result of this attack was the death of most groups involved. In the Sharpe series it results in him being promoted at least once. Going beyond the fictional account I am curious as to what the origination on this is.
Amplifying Tom Au's answer:
With the military revolution in European warfare, two features entered military operations:
Prior to the military revolution (cf: Tercio), a subcaste of the nobility mastered warfare, and primarily gained benefits by in group status and rapine. However, the professionalisation of military life meant that in group status benefits were no longer suitable rewards. Instead, systems of advancement, ennoblement, and recognition amongst an open set of wealthy professionals came to replace in group status amongst a caste. For the most junior, and poorest officers of newly professional companies, such advancement was eagerly sought—sought well above the chances of survival.
Secondly, the advances in troop density, decisive infantry engagement, and siege weaponry led to significant changes in the structure of siege warfare. Star forts developed, and again this led to the development of professionalisation. As sieges became more a result of art and engineering than chaos and starvation, the "storming" phase of sieges became much more highly developed. In particular, bastions needed to be stormed by light infantry in skirmish to remove their capacity to defend other emplacements during sieges, and so advance the central purpose of the siege. Such attempts to take by storm had extremely high costs, and the coin paid for those paying the highest price was advancement or in the case of men wealth.
The reduction in smaller field warfare reduced the chance men had for rapine and thus abnormal pleasure, where the increase in troop densities caused systematic and planned pillage to replace amateur rapine. Ordinary men faced military careers devoid of the pleasures of flesh and sin, and so sought the opportunity for abnormal wealth in other phases.
The military necessity of storming heavily defended bastions caused by the military revolution; the change in status structures for those in command of battle caused by the professionalisation caused by the military revolution; and, the change in the opportunity for martial pleasure amongst serving men caused (yet again) by the military revolution in Europe all found their focus in the one event: the folorn hope. The first unit through the breach enjoyed peculiar benefits because of their peculiar and abnormal chance of death. But these benefits were so highly esteemed that the position of the folorn hope was eagerly volunteered for.
By the time of the "Sharpe" fiction series, these cultures had become institutionalised even though warfare had substantially moved back towards field manoeuvre (this change again caused by the continuing effects of the military revolution). While sieges were not central to warfare any more, the folorn hope still played a cultural role related to its military role.
(I say pity more the second unit through the breach: a similar volume of fire, but no advancement).
A "forlorn hope" is a small breach, or at least a weakening of the walls or defense system of a besieged city or fortress. It is the place where the attacker will initially try to enter the defenses, which is why the defenders will typically do their utmost to contain it. As a result, most of the attackers, at least the early ones at the "forlorn hope," will get killed.
Often, this effort is NOT in vain, because while the defenders are trying to contain/repair the breach at the forlorn hope, their attention and manpower will be drawn away from other potential invasion sites. It's possible that the defenders will (initially) contain the breakthrough from the forlorn hope, and succumb to an attack from another direction.