Tang is meanwhile facing another serious threat—Liu Wuzhou, now determined to march south against Tang.

Emperor Gaozu sends Pei Ji against Liu's advancing army, but Pei is defeated by Liu, who then puts Taiyuan under siege.

Li Yuanji flees back to Chang'an, and much of modern Shanxi is seized by Liu.

Emperor Gaozu then sends Li Shimin against Liu, and by summer 620, Li Shimin has defeated Liu, forcing him to flee to the Eastern Turks.

Liu's territory is incorporated into Tang.

Around the same time, however, Dou Jiande the Prince of Xia makes a major offensive against the cities that had submitted to Tang in modern Hebei and Henan, north of the Yellow River, seizing nearly all of them and taking captive Emperor Gaozu's cousin Li Shentong the Prince of Huai'an, Emperor Gaozu's sister the Princess Tong'an, and Li Shiji's father Li Gai.

With Li Gai in Dou's custody, Li Shiji surrenders to Dou as well.

In 620, Li Shiji, in association with another Tang general who had surrendered to Dou, Li Shanghu, plots to ambush Dou, but the plot is discovered; Li Shanghu is killed, and Li Shiji flees back to Tang.

Li Fuwei, in 620, captures much of the lower Yangtze River territory of another agrarian ruler, Li Zitong, the Emperor of Wu, in the name of the Tang Dynasty.

Li Zitong, in turn, defeats and takes over the territory of Shen Faxing the Prince of Liang, roughly modern Zhejiang.

Emperor Gaozu, delegating control of his military and civil administration in the east to Li Shimin, concentrates on reforming coinage and taxation.

Weltenburg Abbey, founded by Irish or Scottish Benedictine monks in about 620 and held to be the oldest monastery in Bavaria, is situated on a peninsula in the Danube, on the so-called "Weltenburg Narrows" or the "Danube Gorge".

The Persians have pushed the imperial forces back to Constantinople by 620.

The great days of the Persian Achaemenid Empire seem to have come again, and there is little in the recent history of Constantinople's emperors that encourages Heraclius to place much faith in the future.

He clearly cannot hope to survive unless he keeps under arms the troops he has brought with him; yet, the fate of Maurice demonstrates that this will be no easy task, given the empire's lack of financial and agricultural resources.

He now begins a campaign against Persia to retake the territories conquered by Khosrau.

Khosrau II captures Ancyra, an important imperial military base in central Anatolia.

Khosrau, in completing the conquest of Egypt and Palestine, has restored the Persian Empire as it existed in 490 BCE under Darius I.

Li Shimin, after defeating Liu, had started a campaign against Wang's Zheng state in fall 620.

He initially could not decisively defeat Zheng, but by spring 621 has put the Zheng capital Luoyang under a tight siege, although he is not able to capture it.

Wang seeks aid from Dou.

The latter agrees, concerned that a Tang victory over Zheng would also mean his own demise, but at the same time is eager to exploit the weakness of the Zheng and claim its domains for himself.

Emperor Gaozu is initially fearful that Dou and Wang would be able to sandwich Li Shimin's forces between them and orders Li Shimin to retreat, but upon Li Shimin's petition changes his mind and permits Li Shimin to remain in the Luoyang region.

Li Shimin, leaving Li Yuanji in charge of the siege of Luoyang, advances and takes up a position at Hulao Pass.

On May 28, 621, the Tang and Xia forces engage at Hulao, and Li Shimin defeats Dou, capturing him.

Despairing, Wang also surrenders, and most of the Zheng territory is seized by the Tang.

The Tang also seize Xia territory, but after Emperor Gaozu executes Dou, Dou's general Liu Heita rises against the Tang and seizes most of the former Xia territory, while …

…Xu Yuanlang, a rebel leader who had previously submitted to Zheng, also rises in revolt, occupying the modern Shandong region.