Nikephoros has conducted successful campaigns during 807-809 against the Bulgarians, who are harassing the empire’s northern frontiers.

Despite the imperial successes, Bulgarian khan Krum seizes Serdica in 809 and incorporates the former imperial possession as a part of the Bulgarian state; it is given the Slav name Sredets (Greek: Triaditsa).

Al-Hakam defeats Hazim ibn Wahb in 809, regaining control in the west.

It will be temporary.

Pope Leo III forbids the addition of Filioque to the Nicene Creed, when asked to confirm the decision of a Council of Aachen held in 809.

Although he approves of the doctrine thus expressed, he also orders that the Nicene Creed, without Filioque, be displayed on silver tablets placed in Saint Peter's Basilica, adding: "Haec Leo posui amore et cautela orthodoxae fidei" (I, Leo, put these here for love and protection of orthodox faith).

Idris II has established the state called, after himself, Idrisid, with the help of Arab refugees coming from both Spain and Aghlabid territory.

By moving the seat of his authority in 809 to Fez, the capital city he had started to build a year earlier, he makes clear he is establishing a state that is distinct from the Awr'ba Berber confederation.

Harun had arranged the division of the Abbasid empire between his sons, a step that accelerates the caliphate’s administrative decline and political disintegration.

In the last days of Harun's life his health is declining and sees in a dream Musa ibn Jafar sitting in a chamber praying and crying, which makes Harun remember how hard he had struggled to establish his own caliphate.

He knows the personalities of both his sons and decides that for the good of the Abbasid dynasty, al-Maʾmūn should be caliph after his death; this he confides to a group of his courtiers before his death on March 24, 809.

One of the courtiers, Fadl ibn Rabi', does not abide by Harun's last wishes and persuades many in the realm that Harun's wishes had not changed.

Later, the other three courtiers of Harun, who had sworn loyalty to Harun by supporting al-Maʾmūn, namely 'Isa Jarudi, Abu Yunus, and Ibn Abi 'Umran find loopholes in Fadl's arguments, and Fadl admits Harun had appointed al-Maʾmūn after him, but, he argues, since Harun was not in his right mind, his decision should not be acted upon.

Al-Maʾmūn is reportedly the older of the two brothers, but his mother is a Persian woman while al-Amin's mother is a member of the reigning Abbasid family.

Fadl ibn al-Rabi causes the army to pledge its allegiance (bay'ah) to Harun's heir al-Amin, who has remained behind in Baghdad.

Amin, who had need of Fadl's experience, sends letters to him urging him to return to the capital, and to bring with him the treasury, which Harun had taken along, as well as the entire expeditionary army assembled to crush the rebellion.

Al-Ma'mun, who is tasked with the governance of Khurasan, regards the withdrawal of the entire army as a betrayal, and vainly tries to dissuade Fadl from this move.

Al-Ma'mun had been vested with the administration of the eastern Khorasan region following his father’s death.

Although Amin acknowledges his brother's almost absolute sovereignty over the eastern provinces of the empire, with his seat at Merv in Khorasan, relations between the brothers soon break down.

Decades of internal political strife had followed the end of the Exarchate of Ravenna in 751 among various settlements vying for supremacy and between pro- and anti-Constantinopolitan factions; attempts by church authorities to acquire temporal influence have also been a factor.

A temporary shift of Venetian loyalties toward the Franks has resulted in somewhat permanent naval conflict in the Adriatic, only interrupted by a truce in 807–808.

The pro-Frankish faction had finally been able to seize power under Obelerio degli Antoneri in 804.

Obelerio and his brother Beato have brought Venice into the orbit of the Carolingian Empire.

However, by calling in Charles's son Pepin, rex Langobardorum, to his defense, Obelerio raises the ire of the populace against himself and his family and they are forced to flee during Pepin's siege of Venice.

The siege proves a costly Carolingian failure.

It lasts six months, with Pepin's army ravaged by the diseases of the local swamps and eventually forced to withdraw.

A few months later Pepin himself will die, apparently as a result of a disease contracted here.

Al-Ma'mun, open to new movements of thought and outside influences, courts the support of Iranian figures and of the eastern provinces.

Krum’s victory at Serbia has provoked Nikephoros to settle Anatolian populations along the frontier to protect it and to attempt to retake and refortify Serdica, although this enterprise ultimately fails.

Al-Amin, in contrast to his brother, emphasizes traditionalism and Arab culture.

Back in Baghdad, Fadl remains Amin's leading advisor, but his role in the governance of the state seems to have been limited.

Nevertheless, he is the leading figure among those in the Abbasid establishment who pressure Amin into reversing his father's succession plans and depriving Ma'mun of his Khurasani governorate as well as his place in the succession in favor of Amin's son Musa.

This policy increases the already existing polarization of the Abbasid elites between the two princes, with the Khurasani nobility, headed by Ma'mun's wazir al-Fadl ibn Sahl, flocking to Ma'mun, whom they see as the champion of their interests against the central government in Baghdad.

The breach between the two sides is complete in November 810, when Amin drops Ma'mun's name from the Friday prayer.

This leads to a chain of mutual acts that result in the outbreak of open civil war (the "Fourth Fitna") between the two brothers.