After Li Yuan raised an army in Taiyuan, he appointed himself as a great general and actively attacked the Sui army. At this time, the Tujue (Turks) were very powerful. Li Yuan, on one hand worried about the Tujue causing trouble, and on the other hand wanting to borrow their fierce horses, followed advice to seek peace with Shibi Khan of the Tujue. They agreed that 'if we succeed in attacking Chang'an, the people and land will belong to Tang, while gold, jade, and silk will belong to the Tujue.' To win public support, Li Yuan opened granaries to relieve disaster victims and seized the opportunity to recruit volunteers. However, these volunteers were a motley crew, untrained and uninspected, making them extremely difficult to lead.

Meanwhile, Li Mi had established a formidable force in eastern Henan and issued a famous proclamation denouncing Emperor Yang of Sui. A renowned line from this proclamation stated: 'Even if all the bamboo from the Southern Mountains were used, his crimes would be endless; even if the waves of the East Sea were unleashed, his wickedness could not be fully washed away.' Here, '罄' means 'to exhaust' or 'to use up completely.' This line signifies that Emperor Yang's crimes were so numerous that they could not be fully written down even if all the bamboo for writing was exhausted, nor could his evil deeds be cleansed even by all the water of the East Sea. This is the origin of the idiom '罄竹难书,' which we now use to describe crimes or wrongdoings that are too numerous to be recorded.

After this proclamation was issued, it caused a sensation across the land, and everyone circulated it. Li Mi's prestige soared, and various anti-Sui leaders, such as Dou Jiande and Xu Yuanlang, successively petitioned him to assume the imperial throne. However, Li Mi believed that Luoyang had not yet been captured, so there was no need to rush.

Because of Li Mi's great influence, Li Yuan sought to win him over and sent an envoy with a letter. Li Mi, confident in his own strength, demanded that Li Yuan lead several thousand cavalry and infantry to Henan to conclude an alliance in person, with Li Mi serving as the alliance leader. Li Yuan did not dare to offend Li Mi but also did not want to travel to Henan. He smiled and said, 'This Li Mi is so boastful and arrogant. I am preparing to advance into Guanzhong. If I flatly refuse him, it would mean adding another enemy for no reason. It's better to flatter him, make him even more arrogant, and then slowly watch the snipe and clam fight, so I can reap the benefits.'

Thus, Li Yuan ordered Wen Daya to write a reply to Li Mi. The letter stated, 'Heaven gives birth to all people, and there must be a ruler for them. Who else but you could be the ruler, the Son of Heaven, in these times? This old man, past the age of fifty, has no such ambition. I am willing to follow you, my younger brother, and attach myself to your greatness.' Upon reading the letter, Li Mi was overjoyed and laughed heartily. From then on, he completely trusted Li Yuan.