中华上下五千年吴三桂借清兵的故事
公元1644年，李自成在西安正式建立了政权，国号大顺。接着，李自成率领一百万起义将士，渡过黄河，分两路进攻北京。两路大军势如破竹，在这年三月，就在北京城下会师。城外驻守的明军最精锐的三大营全部投降。
起义军猛攻北京城。在第二天晚上，崇祯帝登上煤山（今北京景山）上往四周一望，只见火光映天，知道形势危急，再次回到宫里，拼命敲钟，不料连个人影儿都没有。这时候，他才知道末日到来，又回到煤山，在寿皇亭边一棵槐树下上吊。统治中国二百七十七年的明王朝宣告灭亡。
大顺起义军攻破北京，大将刘宗敏首先率领队伍进城，其后，大顺王李自成头戴笠帽、身穿青布衣、跨着骏马缓缓地进了紫禁城。北京的百姓像过节一样张灯结彩欢迎起义军。大顺政权一面出榜安民，一面严惩明王朝的皇亲国戚、污吏。
有个大官僚吴襄，也被刘宗敏抄了家产，并且逮捕起来追赃。一位高级官员向李自成建议说，如果把吴三桂招降了，那么不仅可以解除了一个威胁，还能加强自己的力量。这位高级官僚写了一封信给他的儿子，让他劝说父亲向起义军投降。
Wu Sangui was originally appointed by the Ming government to defend against the Manchu invasion in Liaodong. He had received orders from the Chongzhen Emperor to lead his troops into Beijing, but when he heard that Beijing had already fallen, he was hesitant. After receiving a letter from his father urging him to surrender, Wu Sangui decided to retreat back to Liaodong.
However, Li Zicheng and his army were not willing to let go of this opportunity. They sent a second message asking for Wu Sangui's surrender again. This time, Wu Sangui refused once more.
Li Zicheng then led an army of over 20 million soldiers towards Liaodong himself. When they arrived at Shanhai Pass, they surrounded Wu Sangui's forces on all sides. Despite being outnumbered and outgunned, Wu Sangui fought bravely alongside his men until they were defeated by the sheer force of Li Zicheng's army.
In desperation, Wu Sangui turned to the Manchus for help and requested their intervention in exchange for loyalty and assistance in defeating Li Zicheng's forces.
The Qing Dynasty took advantage of this situation and agreed to send reinforcements under Prince Dorgon (Dorhgon) who would arrive with 10-15 thousand soldiers within days after receiving news of Wusanguis request.
When Dorhgon arrived at Shanhai Pass with reinforcements on May 27th 1644 AD., Wusanguis forces were vastly outnumbered but still held off against Li Zhicongs main force while waiting for Dorhgons arrival.
When Dorhgon finally made contact with Wusanguis forces outside Shanhai Pass on June 18th 1644 AD., it was clear that there would be no victory without further reinforcement.
On June 29th 1644 AD., just before sunrise as a fierce storm rolled in which prevented both sides from seeing each other clearly during intense fighting between both armies amid heavy rainstorm conditions; it was then that Prince Dorgon ordered an all-out attack on Zhicongs main force while simultaneously having part of his own troops surround Zhicongs outer guard units; leading up to a final decisive battle whereupon Zhang Xianzhongs loyalist soldiers became increasingly disheartened due mainly because their leader Zhang Xianzhong abandoned them at critical moment leaving them stranded behind enemy lines without any hope left.
