Investment (military) in the context of "Sicilian Expedition"

In 717–718, Constantinople, the capital of the Byzantine Empire, was besieged by the Muslim Arabs of the Umayyad Caliphate. The campaign marked the culmination of twenty years of attacks and progressive Arab occupation of the Byzantine borderlands, while Byzantine strength was sapped by prolonged internal turmoil. In 716, after years of preparations, the Arabs, led by Maslama ibn Abd al-Malik, invaded Byzantine Asia Minor. The Arabs initially hoped to exploit Byzantine civil strife and made common cause with the general Leo III the Isaurian, who had risen up against Emperor Theodosius III. Leo, however, deceived them and secured the Byzantine throne for himself.

After wintering in the western coastlands of Asia Minor, the Arab army crossed into Thrace in the early summer of 717 and built siege lines to blockade the city, which was protected by the massive Theodosian Walls. The Arab fleet, which accompanied the land army and was meant to complete the city's blockade by sea, was partly neutralized soon after its arrival by the Byzantine navy through the use of Greek fire. This allowed Constantinople to be resupplied by sea, while the Arab army was crippled by famine and disease during the unusually hard winter that followed. In spring 718, two Arab fleets sent as reinforcements were destroyed by the Byzantines after their Christian crews defected, and an additional army sent overland through Asia Minor was ambushed and defeated. Coupled with attacks by the Bulgars on their rear, the Arabs were forced to lift the siege on 15 August 718. On its return journey, the Arab fleet was almost completely destroyed by natural disasters.

The siege of Baghdad took place in early 1258. A large army commanded by Hulegu, a prince of the Mongol Empire, attacked the historic capital of the Abbasid Caliphate after a series of provocations from its ruler, caliph al-Musta'sim. Within a few weeks, Baghdad fell and was sacked by the Mongol army—al-Musta'sim was killed alongside hundreds of thousands of his subjects. The city's fall has traditionally been seen as marking the end of the Islamic Golden Age; in reality, its ramifications are uncertain.

After the accession of his brother Möngke Khan to the Mongol throne in 1251, Hulegu, a grandson of Genghis Khan, was dispatched westwards to Persia to secure the region. His massive army of over 138,000 men took years to reach the region but then quickly attacked and overpowered the Nizari Ismaili Assassins in 1256. The Mongols had expected al-Musta'sim to provide reinforcements for their army—the Caliph's failure to do so, combined with his arrogance in negotiations, convinced Hulegu to overthrow him in late 1257. Invading Mesopotamia from all sides, the Mongol army soon approached Baghdad, routing a sortie on 17 January 1258 by flooding their opponents' camp. They then invested Baghdad, which was left with around 30,000 troops.

A siege (from Latin sedere '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. The attacking party is said to be laying siege. 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, or mining (also known as sapping), or the use of deception or treachery to bypass defenses.

The Battle of Alesia or Siege of Alesia (September 52 BC) was the climactic military engagement of the Gallic Wars, fought around the Gallic oppidum (fortified settlement) of Alesia in modern France, a major centre of the Mandubii tribe. It was fought by the Roman army of Julius Caesar against a confederation of Gallic tribes united under the leadership of Vercingetorix of the Arverni. It was the last major engagement between Gauls and Romans, and is considered one of Caesar's greatest military achievements and a classic example of siege warfare and investment; the Roman army built dual lines of fortifications—an inner wall to keep the besieged Gauls in, and an outer wall to keep the Gallic relief force out. The Battle of Alesia marked the end of Gallic independence in the modern day territory of France and Belgium.

Archeological and historical research confirms that the battle site was most probably atop Mont Auxois, above modern Alise-Sainte-Reine in France.

During the Reconquista, the siege of Córdoba (Spanish: Conquista de Córdoba, Conquest of Córdoba) was a successful investment by the forces of Ferdinand III, king of Castile and León, marking the end of the Islamic rule over the city that had begun in 711.