In 1402, Zhu Di the King of Yan invaded Nanjing and took away the throne of his nephew Jianwen Emperor Zhu Yunjiang, becoming the new master of the Ming Empire. Although Zhu Di seized power by strong force, after all, he was not the orthodox successor designated by Zhu Yuanzhang, and the legitimacy of Zhu Di's ruling power was always in doubt, which also indirectly led to Zhu Di's later efforts to govern and expand the territory, and the reason was that he hoped to prove his legitimacy by doing something.
Just when Zhu Di was scratching his ears for his own legitimacy, the grassland units that had previously been subdued by the Ming army in the late Hongwu period took advantage of Zhu Di's battle with the imperial court army to gradually go south to encroach on the Ming army's strongholds along the Great Wall, and the security situation in the northern border of Daming became tense again. In fact, the four-level defense system planned by Zhu Yuanzhang was the most scientific, that is, the Son of Heaven coordinated and commanded the national war in Nanjing; Beiping, Taiyuan and Xi'an served as secondary transit stations and theater command centers; Liaodong, Daning, Datong, Ningxia, Gansu and other important places along the Great Wall as strategic frontiers; The strongholds of Kaiping and Daning north of the Great Wall served as the strategic forward exit of the Ming army on the grassland, monitoring the movements of various parts of the grassland at any time, so that the rear could respond to the battlefield situation in a timely manner. Thanks to Zhu Yuanzhang's perfect four-level defense system, the Ming army and the grassland in the early Ming Dynasty, the Ming army had the advantage, and it was also the era when the security situation in the north was the most superior in the Ming generation.
However, Zhu Di's Battle of Jing completely destroyed the four-level defense system that Zhu Yuanzhang painstakingly managed. First of all, Zhu Di himself was an extremely important link in the four-level defense system, and his defense area at that time was the North China Plain around Beiping, because of the Battle of Jingjian, Zhu Di led the army that was supposed to defend the north to fight the civil war in the south, resulting in the opening of the defense line around Beiping; In addition, in the Battle of Jingjian, in order to strengthen his strength, Zhu Di also designed to entrap 80,000 people under the Ning King in the Daning (Xifengkou) area to fight a civil war in the interior, resulting in the emptiness of Daning's defense, and finally its fall, and the Ming army's strategic offensive stronghold on the grassland fell; Zhu Di, who rebelled and seized power as a clan king, for the sake of his imperial power to be forever consolidated, also began the campaign to reduce the domain after the Battle of Jingyan, he moved all the Sai kings along the Great Wall inward and relieved them of their military power, so that Zhu Yuanzhang's four-level defense system was completely abolished, and the security situation in the north became more and more serious.
The situation in front of Zhu Di is very grim, Zhu Di is impossible to restore Zhu Yuanzhang's four-level defense system, what he is most afraid of now is that his brothers will also have a battle with him, and Zhu Di is not worried about handing over the northern army to the generals, after all, the lessons of the Tang Empire's clan and town rebellion cannot be described as profound. In the end, Zhu Di chose a different path from Zhu Yuanzhang, that is, he personally went up, Zhu Di moved the capital directly to the front line of the battle against the enemy, his own base camp Beiping, changed Beiping to Beijing, as a border city to become the capital of a country, which is rare.
After Zhu Di moved his capital to Beijing, he could command the army near Beijing, firstly, he could respond quickly, and secondly, the emperor personally controlled the northern heavy army group, completely eliminating the possibility of rebellion by the clan king or military generals. Zhu Di organized the garrison near Beijing into a strategic mobile corps, the three battalions of the Beijing Division, and then led this army on five expeditions to Mobei, playing the heroic role of Daming's "Son of Heaven guarding the gate of the country".
Unfortunately, everything has pros and cons, and the national defense strategy of the Son of Heaven guarding the border looks heroic, but it is actually Zhu Di's helpless move. And the technical requirements of this work are extremely high, the battle-hardened Zhu Di can control, but his descendants are difficult to control without his strength, and even counterproductive, such as in 1449, Zhu Di's great-grandson, Ming Yingzong Zhu Qizhen because of imitating Zhu Di's imperial driving conquest, as a result of 200,000 Ming army elite reimbursement in Tumu Fort, Zhu Qizhen himself was captured, the Daming Empire was changed by this, from prosperity to decline. Since then, Beijing has always been under the threat of the northern grasslands, and the Son of Heaven guarding the country has become a very embarrassing "self-mockery" (because it really has the ability, and should not let the capital become a border city).