Snippet from Wikipedia: Siege

A siege (Latin: sedere, lit. 'to sit') is a military blockade of a city, or fortress, with the intent of conquering by attrition, or by well-prepared assault. Siege warfare (also called siegecraft or poliorcetics) is a form of constant, low-intensity conflict characterized by one party holding a strong, static, defensive position. Consequently, an opportunity for negotiation between combatants is common, as proximity and fluctuating advantage can encourage diplomacy.

A siege occurs when an attacker encounters a city or fortress that cannot be easily taken by a quick assault, and which refuses to surrender. Sieges involve surrounding the target to block provision of supplies and reinforcement or escape of troops (a tactic known as "investment"). This is typically coupled with attempts to reduce the fortifications by means of siege engines, artillery bombardment, mining (also known as sapping), or the use of deception or treachery to bypass defenses.

Failing a military outcome, sieges can often be decided by starvation, thirst, or disease, which can afflict either the attacker or defender. This form of siege, though, can take many months or even years, depending upon the size of the stores of food the fortified position holds. The attacking force can circumvallate the besieged place, which is to build a line of earth-works, consisting of a rampart and trench, surrounding it. During the process of circumvallation, the attacking force can be set upon by another force, an ally of the besieged place, due to the lengthy amount of time required to force it to capitulate.

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Snippet from Wikipedia: Battle of Verdun

The Battle of Verdun (French: Bataille de Verdun [bataj də vɛʁdœ̃]; German: Schlacht um Verdun [ʃlaxt ʔʊm ˈvɛɐ̯dœ̃]) was fought from 21 February to 18 December 1916 on the Western Front in France. The battle was the longest of the First World War and took place on the hills north of Verdun-sur-Meuse. The German 5th Army attacked the defences of the Fortified Region of Verdun (RFV, Région Fortifiée de Verdun) and those of the French Second Army on the right (east) bank of the Meuse. Using the experience of the Second Battle of Champagne in 1915, the Germans planned to capture the Meuse Heights, an excellent defensive position, with good observation for artillery-fire on Verdun. The Germans hoped that the French would commit their strategic reserve to recapture the position and suffer catastrophic losses at little cost to the German infantry.

Poor weather delayed the beginning of the attack until 21 February but the Germans captured Fort Douaumont in the first three days. The advance then slowed for several days, despite inflicting many French casualties. By 6 March, 20+1⁄2 French divisions were in the RFV and a more extensive defence in depth had been organised. Philippe Pétain ordered there to be no retreat and that German attacks were to be counter-attacked, despite this exposing French infantry to the German artillery. By 29 March, French guns on the west bank had begun a constant bombardment of Germans on the east bank, causing many infantry casualties.

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