Li Yuan has Emperor Gong yield the…

The short-lived Sui dynasty ends in spring 618 with Li Yuan’s capture of Luoyang and the assassination of Emperor Yang at Jiangdu (in modern Yangzhou, Jiangsu) in a coup led by the general Yuwen Huaji.

The Sui officials at Luoyang meanwhile declare another grandson of Emperor Yang, Emperor Gong's brother Yang Tong, the Prince of Yue, as emperor, and refuse to recognize the regime change in Chang'an.

The weakened Sui dynasty collapses into petty states.

Emperor Gaozu immediately faces a major challenge from Xue Ju, an agrarian leader who has declared himself the Emperor of Qin.

Xue takes advantage of Li Shimin's illness during the fall of 618 to defeat an army commanded by Li Shimin and Liu Wenjing at Qianshui Plain (in modern Xianyang, Shanxi) and approach Chang'an.

In response, Gaozu tries to enter an alliance with Li Gui, the Prince of Liang, between whose domain and the Tang Xue's Qin state was located, writing Li Gui and referring to him as cousin.

Li Gui briefly submits to Gaozu.

Meanwhile, before he can attack Chang'an, Xue Ju dies of illness and is succeeded by his son Xue Rengao, who is a capable commander but who has alienated his generals because of his cruelty.

Li Shimin is soon able to attack Xue Rengao at Gaozhi (in modern Xianyang as well), and force Xue Rengao to surrender.

The Avars send envoys to Constantinople for an meeting with Heraclius, who in an effort to placate the Avars, meets them at Thracian Heraclea, outside the long walls protecting the capital, in 617 or 619.

They seek to capture him, and he, warned of an ambush, gallops madly back to Constantinople, the Avars in hot pursuit.

Angry at the failure to capture the emperor, the Avars plunder Thrace and return to the Danube River carrying off two hundred and seventy thousand people.

The expansionist King Harsha of Thaneswar pushes eastward across the North Indian Plain around 618, meeting no resistance and gaining the allegiance of several petty kings.

Shashanka of Gauda, faced with Harsha’s army of fifty thousand foot soldiers, twenty thousand cavalry, and five thousand elephants, forfeits (either through battle defeat or pragmatic politics; scholars are uncertain) some territory in Pundra-vardhana, which Harsha divides with his ally, Bhaskara Varman of Kamarupa.

A Persian expeditionary force under Shahrbaraz invades Egypt and occupies the province.

The Persians besiege Alexandria, whose defense of the city is led by Nicetas, a cousin of emperor Heraclius.

The imperial resistance is undermined by a blockade of the harbor, which cuts off the usual grain supplies from Egypt to Constantinople.

After the Persians capture the besieged city, Nicetas and the Chalcedonian patriarch, John V, flee to Cyprus, and King Khosrau II extends his rule southwards along the Nile.

Wang Shichong at Luoyang has Yang Tong yield the throne to him in spring 619, ending the Sui dynasty and establishing a new state of Zheng.

Around the same time, Li Gui, while stating that he wishes to be a Tang subject, refuses the Tang creation of Prince of Liang, instead declaring himself the Emperor of Tang.

In summer 619, Li Gui's official An Xinggu, formerly a Tang official, rebels against Li Gui and captures him, submitting to Tang.

Gaozu executes Li Gui and incorporates his domain into Tang.

Also around the same time, …

…Li Mi, having been defeated earlier in the year in a surprise attack by the Sui general Wang Shichong, flees to Tang territory and submits to Gaozu.

Li Mi's general Xu Shiji, who controls a major part of Li Mi's former territory, also submits, and Emperor Gaozu, impressed with Xu's faithfulness to Li Mi, bestows the imperial surname of Li on Xu.

Gaozu creates Li Mi the Duke of Xing, but only makes him the Minister of Feasts, a post that Li Mi views as below his stature.

Around New Year 619, Li Mi requests Emperor Gaozu's permission to head east to persuade some of his former subordinates to submit to Tang, but once he leaves Chang'an, plans to restore his independence.

He is ambushed and killed by the Tang general Sheng Yanshi.

…the rebel leader Du Fuwei, who controls the modern southern Anhui, submits to Tang, and Gaozu also bestows the imperial surname of Li on him, creating him the Prince of Wu.

Similarly, …

…Luo Yi, who controls the modern Beijing region, submits, is bestowed the imperial surname of Li, and is created the Prince of Yan.