The rivalry between Li Jiancheng and Li Shimin has intensified.

Li Jiancheng, to supplement his guard corps, requisitions a number of soldiers from the general Li Yi the Prince of Yan in 624, against Emperor Gaozu's regulations.

When this is revealed to Emperor Gaozu, Emperor Gaozu rebukes Li Jiancheng and exiles his guard commander Keda Zhi.

When, subsequently, Li Jiancheng nevertheless requests the commandant at Qing Prefecture (in modern Qingyang, Gansu), Yang Wen'gan, to conscript troops, presumably to guard against Li Shimin, …

…the officers Erzhu Huan and Qiao Gongshan inform Emperor Gaozu that Li Jiancheng is encouraging Yang to start a rebellion so that they could seize power together.

Emperor Gaozu, then at Renzhi Palace (in modern Tongchuan, Shaanxi), is incensed, and summons Li Jiancheng from Chang'an to Renzhi Palace.

Li Jiancheng briefly entertains the notion of occupying Chang'an and not accepting the order, but eventually reports to Renzhi Palace to request forgiveness.

Emperor Gaozu puts him under arrest.

When Yang hears this, Yang rebels, and Emperor Gaozu, after promising Li Shimin that he would be made crown prince, sends Li Shimin to attack Yang.

(Under Emperor Gaozu's promise, Li Jiancheng will be removed as crown prince and created the Prince of Shu instead.

He will then send Li Jiancheng to the modern Sichuan region.)

Once Li Shimin leaves, however, Li Yuanji, Emperor Gaozu's concubines, and the chancellor Feng Deyi, all speak on Li Jiancheng's behalf, and Emperor Gaozu changes his mind, releases Li Jiancheng, and allows him to return to Chang'an and remain as crown prince.

Instead, Emperor Gaozu only blames the discord between his sons on Li Jiancheng's staff members Wang Gui and Wei Ting, and Li Shimin's staff member Du Yan, exiling them.

Yang is subsequently assassinated by his own subordinates.

Heraclius, hoping in 624 for a compromise that might be acceptable to the Monophysites of Egypt and Syria, adopts the Monothelitic formula, which accepts the teaching of two natures in Jesus Christ, but declares he had only one will or mode of activity (energeia).

Sergius, patriarch of Constantinople, champions the doctrine and gains the support of the Egyptian and Armenian Monophysites.

Heraclius advances with an expeditionary force (forty thousand men) along the Araxes River, destroying the fortress city of Dvin, capital of Armenia, and …

…Nakhchivan (modern Azerbaijan).

Heraclius has devoted two years to campaigns in Armenia, the manpower of which is vital to the empire, fighting together with allies found among the Christian Caucasian peoples: …

…the Abasgi, …

…the Iberians, and …

…the Lazi.

Heraclius defeats the Persian army at Ganzak and destroys the fire temple and sanctuary of Adur Gushnasp.

He winters his army in Caucasian Albania to gather forces for the next year.

Khosrau, withdrawing most of his troops from Chalcedon, assembles three armies to trap and destroy Heraclius' forces.

The Persians go into winter quarters nearby, but Heraclius attacks them at Tigranakert in Western Armenia.

Routing the forces of the generals Shahin Vahmanzadegan and Shahraplakan, the imperial forces destroy their baggage train.