Al-Muhasibi of Baghdad (Abu Abd Allah al-Harith ibn Asad al-Muhasibi al-Anazi; literally, "he who examines his conscience"), a member of the Shafiite school of Muslim legal thought, is caught up in a persecution of the Mutazilites by leaders of the rival Hanbalite school of jurisprudence, and is compelled to give up his teaching in 846.

In his main work, “Ar-ri ayah li-huquq Allah” (“Book of Observance of What Is Due to God”), al-Muhasibi argues that asceticism should be tempered by attention to the inner and outer duties toward God.

He proposes the use of the rationalist method of rigorous self-examination in anticipation of the Last Judgment (“muha sabah”) to achieve moral purification.

He employs the dialectic vocabulary of the Mutazilites, the founders of Muslim scholasticism, but differs with several of their doctrines.

Sergius contributes to urban redevelopment in Rome, relying on what some consider to be dubious means of gaining money.

Starting in 827, Muslim forces had begun to conquer Sicily.

The Saracens of southern Italy, repulsed at Naples in 846, come north to the mouth of the Tiber River, capture the fort recently constructed there by the late Pope Gregory, and march on Rome, looting and burning the suburbs as they advance.

They reach the Vatican Hill area outside Rome’s defensive walls, and pillage the buildings there, desecrating Saint Peter’s Basilica, then, laden with plunder, escape to their ships.

The Saracens also assault Portus and Ostia in August 846.

Ramiro I’s Failed Repopulation of León and the Muslim Counterattack (846 CE)

While King Ramiro I of Asturias seeks to repopulate León, his most significant military confrontation with the Muslim emirates of Iberia proves unsuccessful.

At the same time, Emir Abd ar-Rahman II of Córdoba is occupied with other pressing threats, including:

• Viking invasions, which had reached Lisbon and Seville, and
• Internal rebellions, particularly those led by Musa ibn Musa of the Banu Qasi family, a powerful Muladi(Muslim of Iberian descent) ruler in the Ebro Valley.

Seizing the opportunity, Ramiro resettles León, attempting to reclaim and secure an area that had been left largely uninhabited for nearly a century.

The Desert of the Duero and the Christian-Muslim Frontier

During the reign of Alfonso I of Asturias (739–757), Christian forces had successfully retaken Galicia, La Rioja, and León through a series of military campaigns. However, rather than holding these lands, they had evacuated much of the population to the north, leaving a vast, largely uninhabited buffer zone between Christian Asturias and Muslim Al-Andalus, known as the Desert of the Duero.

For nearly a century, this depopulated region had served as a natural barrier, discouraging large-scale invasions. But Ramiro’s attempt to resettle León brings renewed Muslim attention to the frontier.

The Muslim Counterattack and the Destruction of León (846 CE)

By 846 CE, Emir Abd ar-Rahman II has dealt with both Viking raiders and internal revolts. With his rule stabilized, he dispatches an army led by his son and future successor, Muhammad I of Córdoba.

The Muslim forces quickly overwhelm León, forcing the Asturian settlers to evacuate. After reclaiming the city, the Muslims burn León to the ground, ensuring that it remains uninhabited for another decade.

León’s Later Resettlement

The city is not permanently reoccupied until 856 CE, under the reign of King Ordoño I of Asturias, when the Asturians once again attempt to consolidate their presence in the region.

Ramiro I’s short-lived repopulation effort underscores the continued fragility of the Christian-Muslim frontier and the long struggle to reclaim the Duero Valley, which remains a contested zone well into the 10th century.

The Peace Agreement Between Charles the Bald and Nominoe (846): The Recognition of Breton Autonomy

Following his defeat at the Battle of Ballon (845), Charles the Bald marches on Brittany again in the summer of 846. However, instead of engaging in further military action, he negotiates a peace settlement with Nominoe, the de facto ruler of Brittany.

The Terms of the Peace Agreement

While the exact details of the peace settlement remain uncertain, historical sources suggest:

• Nominoe and Charles exchange oaths, likely formalizing Breton autonomy under nominal Frankish overlordship.
• Prudentius of Troyes refers to Nominoe as "Duke" (dux) of the Bretons, possibly indicating that Charles officially recognizes Nominoe as ruler in exchange for his acknowledgment of Frankish suzerainty.
• As part of the arrangement, Nominoe demands the removal of Lambert II of Nantes, a Frankish noble who had been an opponent of Breton interests.
• Charles removes Lambert from Nantes and relocates him to another region, weakening his influence over Breton affairs.

Significance of the 846 Agreement

• While Charles nominally retains overlordship, this agreement effectively solidifies Nominoe’s independent rule over Brittany.
• By securing Lambert’s removal, Nominoe eliminates a powerful Frankish rival, further consolidating his authority.
• This diplomatic resolution temporarily ends the Frankish-Breton conflict, though tensions will later resurface as Breton expansion into Frankish territory continues.

Aftermath and the Path Toward Breton Sovereignty

• In the following years, Nominoe continues to strengthen Brittany, even launching further campaigns into Frankish lands.
• By 850, Brittany will no longer be merely a semi-autonomous duchy but will begin functioning as a fully independent entity, defying Carolingian control.

Thus, while the 846 settlement represents a temporary peace, it is a major step toward Breton independence, marking the beginning of Brittany’s long resistance against Frankish rule.

The late Nikephoros, deposed as patriarch of Constantinople for his anti-iconoclast position, is canonized in 847.

The process of Islamization gains added momentum under the 'Abbasids as a result of further restrictions imposed on non-Muslims, particularly by al Mutawakkil, who, when he succeeds al-Wathiq as caliph in 847, reverts to a position of Islamic orthodoxy and begins a persecution of all non-orthodox or non-Muslim groups.

Synagogues and churches in Baghdad are torn down or converted into mosques, while ...

...in Karbala', the shrine of al-Husayn ibn 'Ali, the first saint of the Shi'a Muslims, is razed, and further pilgrimages to the town are forbidden.

Sergius dies in January 847 while negotiating between two patriarchs.

The cardinal of Santi Quattro Coronati, Roman by birth, a former subdeacon of Gregory IV and archpriest under Sergius, is unanimously chosen as his successor, elected on April 10, 847.

The pontificate of Leo IV will chiefly be distinguished by his efforts to repair the damage done by the Saracens during the reign of his predecessor to various churches of the city, especially those of St Peter and St Paul.

The Spanish Muslim pirates called Saracens set up small Muslim states on Italy’s southern coast at Taranto and Bari, using these as bases from which to ravage the Adriatic and Tyrrhenian seacoasts, often raiding inland.

Kalfun, of Berber descent, hails from the Aghlabid emirate in North Africa; a former servant, and possibly formerly enslaved, he had been part of the mercenary garrison installed in Bari, by Radelchis I of Benevento, In 847, Kalfun captures the formerly imperial city of Bari and sets himself up as the emir in this city, hereby founding the Emirate of Bari, which will last until 871.