明末清初的学者顾炎武(1613-1681年)在论及几个王朝覆灭前发生在居庸关一带的战事时，感慨地说：“地非不险，城非不高，兵非不多，粮非不足也，国法不行而人心去也。”人心一去，纵有长城漫漫、雄关重重，又哪里能守得住风雨飘摇的江山呢?
用长城作屏障，早在春秋(前722年-前481年)诸国争霸的时候就有了，为了防御入侵，各诸侯国纷纷大兴土木，相继建起了长城，但最终都未能依仗长城而逃脱被秦国翦灭的命运。
秦始皇(前221-前210年在位)一统宇内，自以为功盖三皇，业冠五帝，于是忘乎所以，令蒙恬北击匈奴，督修万里长城，以防“胡”灭秦。最终的结局却是，长城修完了，秦朝的江山社稷也瓦解了。
自秦朝以后，几乎每朝每代都在维修和扩建长城，梦想以此来巩固自己的宝座。可从来就没有哪位真龙天子能够如愿以偿。
北魏明元帝(409-423年在位)泰常八年(423年)，整修了东起赤城(今河北赤城县)西至五原(今内蒙古包头市西北)的长城二千余里，并在沿线要地设置六个军事重镇。可是，长城尚未修完，这六个地方就已经燃起了熊熊烈火，但不是长城抵御外敌的狼烟，而是六镇兵民起义的烽火。这段专制者倒台的历史虽与长城有关，但在数千年波翻浪涌的历史长河中不过是几圈不起眼的涟漪。
修长城最卖力的是明太祖朱元璋(1368-1398年在位)的子孙们，他们历经270余年，对长城进行了18次大规模修复，几乎是在秦长城的基础上重修了一遍，构成了“层层布防”的纵深防御体系，沿线陈兵90余万，称得上“寸土设障，步步为营”。在重要的关口，尤其是在当时的都城-北京居庸关一带，层层筑起坚实高大的城墙。然而，这看起来固若金汤的长城防线，也数度失守。明正统十四年(1499年)，瓦剌首领也先率众攻破长城，在土木堡(今河北怀来东)俘虏了“御驾亲征”的英宗皇帝朱祁镇(1436-1449、1457-1464年在位)。嘉靖二十九年(1550年)，鞑靼首领俺答又率众攻入蓟镇地区，从古北口向南，直抵北京城下。1644年，李自成率领的农民起义军挥师突破居庸关，几天后便攻进北京城，宣告了明王朝的覆灭。
What the Great Wall Could Not Do
Gu Yanwu (1613-81), a scholar active in the late Ming and early Qing dynasties, said in commenting on the warfare in the vicinity of the Juyong Pass on the Great Wall following the demise of several dynasties:
“It was not that the terrain was not precipitous, nor was the wall not tall enough; it was not that there were not enough soldiers or the grain supply was inadequate. The rule of the country, however, could not last long because the rulers had lost the heart of the people.”
Once losing the support of the people, rulers could not prevent the fall of their dynastic rule, despite the long and towering defensive work in the form of the Great Wall.
The practice of using the wall as a defensive work began during the period when warring states contended with and tried to annex one another during the Spring and Autumn Period (772-481 BC). They all had defensive walls built but in the end none of them were able to use the wall to keep off the wave of soldiers of the State of Qin.
The First Emperor of Qin who ruled from 221 to 210 BC, once becoming the national ruler by defeating all other states, lost the sense of who he really was and thought he was greater than all previous rulers. He ordered General Meng Tian to fight the northern tribe of the Xiongnu and had a very long wall built to defend against the possible onslaught of the Xiongnus. The outcome was that the completion of the Great Wall was accompanied by the fall of the Qin Dynasty (221-206 BC).
After the Qin, almost every ensuing dynasty maintained, repaired and expanded the Great Wall in the hope of consolidating its rule. None of the “sons of heaven,” however, had the good fortune of achieving his goal.
Emperor Mingyuan, who ruled the Northern Wei from 409 to 423, launched the most massive construction of the wall in 423 when he had more than 1,000 kilometers of the wall repaired or built between present-day Chicheng in Hebei Province in the east to Baotou in Inner Mongolia in the west. He reinforced the system of defense with the establishment of six major military towns along this section. Soon, fire, however, was lit in these six strategic towns, not to signal a fight against northern invaders but to sound the battle cry of rebellious soldiers and residents.
Though the fall of the Northern Wei ruler had something to do with the Great Wall, it was nevertheless a rather small incident during the long history associated with the wall. The most enthusiastic wall builders were the sons and grandsons of the founding emperor of the Ming Dynasty Zhu Yuanzhang (r. 1368-98). During a period of 270 years they launched massive repairs of the Great Wall on 18 occasions, virtually turning it into an entirely new wall and an impenetrable system of defense guarded by some 900,000 troops. “Every inch of land was guarded and every step was protected” was an apt description. Strategic mountain passes, particularly the Juyong Pass that guarded the entrance to the capital city of Beijing, was reinforced with several layers of thick and towering walls. The defense seemed secure, but the fact is that the wall was lost to the enemy several times. In 1499, the northern invaders while attacking the Great Wall captured none other than Emperor Yingzong (r. 1436-49; 1457-1464), who had come to the front to personally lead the defense. Then in 1550, the ancient Tartars, under the command of Anda, fought their way through the Gubei Pass to the city wall of Beijing. On March 15, 1644, Li Zicheng, leader of a peasant rebellion, broke the defense of the Ming troops at the Juyong Pass and three days later, entered Beijing, putting an end to the Ming Dynasty.
