During the Three Kingdoms period, in the first year of Emperor Ming of Wei's reign, a Shu general named Meng Da defected to Wei, and Emperor Ming treated him generously. However, Sima Yi argued, "This man's words and deeds are extraordinary and cannot be trusted," and strongly opposed keeping him. Yet the emperor ignored the advice, appointing Meng Da as governor of the border region Xincheng, granting him a marquis title, and even allowing him to travel with imperial credentials. Seizing every opportunity, Meng Da secretly reconnected with Shu, allied with Wu, and quietly prepared a coordinated attack on Wei.
Zhuge Liang, the Shu chancellor, remained wary of Meng Da, fearing his fickle nature might cause delays or even backfire into disaster. Meng Da had a feud with Shen Yi, another Wei prefect. To force Meng Da's hand, Zhuge Liang sent Guo Mo to feign surrender to Shen Yi, deliberately leaking Meng Da's secret plan to ally with Shu and Wu against Wei. Seeing his plot exposed, Meng Da decided to act without hesitation, preparing to raise his army against Wei immediately.
The situation caught Wei off guard. Sima Yi knew he had to delay Meng Da to buy time, or face disaster. He devised a plan, sending Meng Da a letter feigning comfort: "General Meng, you once abandoned Liu Bei and entrusted everything to Wei. Wei gave you the crucial task of guarding the border, trusting your loyalty. Wei's sincerity toward you is as clear as the sun—truly 'a heart piercing the daylight.' In truth, the people of Shu, both foolish and wise, gnash their teeth at your betrayal. Zhuge Liang seeks a way to defeat you but finds no path. Guo Mo's claim that you allied with Wu and Shu against Wei is no trivial matter. If it were true, why would Zhuge Liang treat it so lightly, letting someone like Guo Mo know and leak it? This is surely easy to see through." Reading this, Meng Da breathed a sigh of relief, feeling he had no need to rush into action, and hesitated over whether to immediately raise his troops.
Thus Sima Yi bought himself precious time. He secretly and swiftly mobilized his troops, making urgent preparations to attack Meng Da. When some generals suggested observing Meng Da further, Sima Yi firmly rejected the idea, ordering his army to march day and night. They covered the distance in just eight days, arriving at the walls of Meng Da's city. Wu and Shu hastily sent forces to aid Meng Da, but Sima Yi's army intercepted them all. Earlier, Meng Da had written to Zhuge Liang, "Sima Yi's garrison is 800 li from the capital Luoyang, and 1,200 li from my position. Upon hearing of my rebellion, he must first report to the capital; the round trip alone would take at least a month." Yet in only eight days, Sima Yi's army was at his gates—Meng Da never dreamed his enemy could move with such lightning speed.
Despite Meng Da's stronghold at Shangyong being protected by water on three sides and reinforced with wooden palisades, Sima Yi's troops crossed the river, broke through the barriers, and attacked the city from eight directions with unstoppable momentum. In just sixteen days, Meng Da's situation was hopeless. Overwhelmed by Sima Yi's military might, Meng Da's own nephew and others were forced to open the city gates and surrender. Meng Da was captured and executed.
Later, the idiom "Heart Piercing the White Sun" came to describe a sincere heart that connects with the sun itself—open, upright, pure, and honest.
Source: *Book of Jin*, "Biography of Emperor Xuan"
Meaning of the Idiom: Later, the Chinese idiom "心贯白日" came to describe a sincere heart that connects with the sun itself—open, upright, pure, and honest.