太和五年(nian)（370年(nian)），前燕(yan)被前秦所滅亡后，包括慕(mu)容(rong)(rong)(rong)(rong)沖(chong)(chong)(chong)及其(qi)兄慕(mu)容(rong)(rong)(rong)(rong)泓(hong)在(zai)內的(de)(de)眾(zhong)多鮮卑慕(mu)容(rong)(rong)(rong)(rong)部人(ren)被遷(qian)往關(guan)中。太元九年(nian)（384年(nian)），慕(mu)容(rong)(rong)(rong)(rong)沖(chong)(chong)(chong)之叔慕(mu)容(rong)(rong)(rong)(rong)垂叛秦建立后燕(yan)，慕(mu)容(rong)(rong)(rong)(rong)泓(hong)亦于關(guan)中舉兵自稱(cheng)濟北王(wang)。時任(ren)平陽太守的(de)(de)慕(mu)容(rong)(rong)(rong)(rong)沖(chong)(chong)(chong)在(zai)河(he)東起兵，擁(yong)有二萬兵眾(zhong)，進(jin)軍(jun)攻(gong)打蒲坂，苻堅令竇(dou)沖(chong)(chong)(chong)征討他。竇(dou)沖(chong)(chong)(chong)在(zai)河(he)東攻(gong)擊慕(mu)容(rong)(rong)(rong)(rong)沖(chong)(chong)(chong)，大敗慕(mu)容(rong)(rong)(rong)(rong)沖(chong)(chong)(chong)，慕(mu)容(rong)(rong)(rong)(rong)沖(chong)(chong)(chong)率騎兵八(ba)千投奔其(qi)兄慕(mu)容(rong)(rong)(rong)(rong)泓(hong)軍(jun)中。
不(bu)久慕(mu)容(rong)(rong)(rong)泓進(jin)軍(jun)長安，建立西燕政權。太(tai)元九年（384年）六月，慕(mu)容(rong)(rong)(rong)泓的(de)謀臣(chen)高蓋、宿勤(qin)崇等認為(wei)慕(mu)容(rong)(rong)(rong)泓的(de)德望不(bu)如慕(mu)容(rong)(rong)(rong)沖(chong)，而(er)且執法苛嚴，于是(shi)就殺了慕(mu)容(rong)(rong)(rong)泓，立慕(mu)容(rong)(rong)(rong)沖(chong)為(wei)皇太(tai)弟，秉承帝旨行事(shi)，自(zi)行設置任(ren)命(ming)官員。 任(ren)命(ming)高蓋為(wei)尚書令。后秦姚萇派遣兒子姚嵩到慕(mu)容(rong)(rong)(rong)沖(chong)處作為(wei)人質，以請求和好。
太(tai)元(yuan)九年（384年）七(qi)月，苻(fu)(fu)堅(jian)聽(ting)說慕(mu)(mu)容(rong)(rong)(rong)沖(chong)離長安二(er)百余(yu)里，就(jiu)(jiu)撤兵(bing)(bing)而回，令撫軍將(jiang)軍苻(fu)(fu)方戍守驪山，拜苻(fu)(fu)暉(hui)為使持(chi)節、散騎常侍、都督(du)中外諸軍事(shi)、車騎大將(jiang)軍、司隸校尉、錄尚(shang)書，配給五萬兵(bing)(bing)力抵御慕(mu)(mu)容(rong)(rong)(rong)沖(chong)，河間公苻(fu)(fu)琳為中軍大將(jiang)軍，作為苻(fu)(fu)暉(hui)的(de)后援部隊(dui)。慕(mu)(mu)容(rong)(rong)(rong)沖(chong)就(jiu)(jiu)命婦(fu)人乘牛馬充數，舉竿子為旗，揚土為灰(hui)塵，督(du)促(cu)勉(mian)勵他(ta)的(de)軍眾(zhong)，早晨(chen)在鄭西進攻(gong)苻(fu)(fu)暉(hui)軍營(ying)。苻(fu)(fu)暉(hui)出(chu)兵(bing)(bing)拒(ju)戰，慕(mu)(mu)容(rong)(rong)(rong)沖(chong)揚塵擊(ji)鼓吶(na)喊，苻(fu)(fu)暉(hui)軍戰敗(bai)(bai)。苻(fu)(fu)堅(jian)又任尚(shang)書姜宇(yu)(yu)為前將(jiang)軍，與苻(fu)(fu)琳率(lv)軍三萬，到灞上(shang)攻(gong)擊(ji)慕(mu)(mu)容(rong)(rong)(rong)沖(chong)，被慕(mu)(mu)容(rong)(rong)(rong)沖(chong)打敗(bai)(bai)，姜宇(yu)(yu)戰死(si)，苻(fu)(fu)琳身中流箭，慕(mu)(mu)容(rong)(rong)(rong)沖(chong)于(yu)是占據并(bing)入住阿(a)房城。
太(tai)元(yuan)九年(nian)（384年(nian)）九月，慕(mu)(mu)容(rong)沖進逼(bi)長安，苻(fu)堅登(deng)上城(cheng)頭觀望，嘆息道：“這(zhe)(zhe)些虜(lu)(lu)賊從(cong)哪里(li)出(chu)(chu)來(lai)的？這(zhe)(zhe)么強盛！”大聲斥責(ze)慕(mu)(mu)容(rong)沖說(shuo)：“你(ni)們這(zhe)(zhe)些群奴(nu)(nu)(nu)正(zheng)好放牧牛(niu)羊，為什么來(lai)送(song)死！”慕(mu)(mu)容(rong)沖說(shuo)：“奴(nu)(nu)(nu)就奴(nu)(nu)(nu)罷，已(yi)經厭(yan)倦了為奴(nu)(nu)(nu)之(zhi)苦，又要(yao)把(ba)你(ni)取(qu)而代之(zhi)。”苻(fu)堅派使(shi)者送(song)一(yi)領(ling)錦(jin)袍給慕(mu)(mu)容(rong)沖，稱詔旨說(shuo)：“古人交戰，使(shi)者在(zai)中間往來(lai)。卿(qing)遠來(lai)諸事草創，能不勞苦嗎？現(xian)贈送(song)一(yi)領(ling)錦(jin)袍，以(yi)表(biao)明(ming)本(ben)心。朕對卿(qing)的恩分怎么樣，而一(yi)夜之(zhi)間出(chu)(chu)現(xian)這(zhe)(zhe)樣的變故！”慕(mu)(mu)容(rong)沖命詹事回(hui)答(da)，亦稱“皇太(tai)弟有令”：“孤現(xian)在(zai)心在(zai)天下，豈能顧念一(yi)領(ling)錦(jin)袍的小恩惠。如果能知道天命，就可以(yi)君臣束手，停止(zhi)交戰，早(zao)點(dian)把(ba)皇帝送(song)出(chu)(chu)來(lai)，自然會(hui)寬(kuan)赦苻(fu)氏(shi)，以(yi)酬(chou)報舊好，終將不讓以(yi)往所(suo)做(zuo)的善事成為沒有可相提(ti)并(bing)論的獨美”。苻(fu)堅大怒說(shuo)：“我不聽(ting)從(cong)王猛(meng)、陽平公之(zhi)言，使(shi)得白虜(lu)(lu)敢到這(zhe)(zhe)樣的地(di)步。”
太元十年（385年）正月(yue)，慕(mu)容(rong)沖(chong)(chong)(chong)(chong)在阿(a)房城稱帝，改年號為(wei)更(geng)始。苻(fu)堅與慕(mu)容(rong)沖(chong)(chong)(chong)(chong)交戰，各有勝(sheng)負(fu)。苻(fu)堅曾被(bei)慕(mu)容(rong)沖(chong)(chong)(chong)(chong)軍(jun)包(bao)圍，前(qian)(qian)秦(qin)殿(dian)中(zhong)上(shang)將(jiang)軍(jun)鄧(deng)邁、左中(zhong)郎(lang)將(jiang)鄧(deng)綏(sui)、尚書郎(lang)鄧(deng)瓊相(xiang)互說：“我(wo)們家世(shi)代蒙(meng)(meng)受榮(rong)耀(yao)寵貴，先君為(wei)國家建立了大(da)功，不(bu)能不(bu)盡忠(zhong)效(xiao)節(jie)，以(yi)繼承先君之(zhi)志。再說不(bu)能為(wei)君主之(zhi)難獻身者，不(bu)是大(da)丈夫。”于是與毛長樂等(deng)蒙(meng)(meng)上(shang)獸皮，揚戈(ge)奮勇攻擊慕(mu)容(rong)沖(chong)(chong)(chong)(chong)軍(jun)，慕(mu)容(rong)沖(chong)(chong)(chong)(chong)軍(jun)潰(kui)敗(bai)。慕(mu)容(rong)沖(chong)(chong)(chong)(chong)又(you)派(pai)他的尚書令(ling)高(gao)蓋率軍(jun)夜襲長安，攻陷(xian)南門(men)，進入(ru)南城。前(qian)(qian)秦(qin)左將(jiang)軍(jun)竇沖(chong)(chong)(chong)(chong)、前(qian)(qian)禁將(jiang)軍(jun)李辯等(deng)擊敗(bai)高(gao)蓋軍(jun)，高(gao)蓋軍(jun)眾被(bei)斬首一(yi)千(qian)八百人(ren)，并被(bei)前(qian)(qian)秦(qin)軍(jun)分尸而(er)吃。接(jie)著苻(fu)堅又(you)在城西打敗(bai)慕(mu)容(rong)沖(chong)(chong)(chong)(chong)，追擊慕(mu)容(rong)沖(chong)(chong)(chong)(chong)的敗(bai)軍(jun)到(dao)阿(a)房城。前(qian)(qian)秦(qin)眾將(jiang)請(qing)求乘勝(sheng)入(ru)城，苻(fu)堅擔心被(bei)慕(mu)容(rong)沖(chong)(chong)(chong)(chong)俘(fu)獲，就(jiu)嗚鑼罷兵。
太元十年（385年）三(san)月，苻(fu)暉(hui)屢次被慕(mu)容沖擊敗，苻(fu)堅(jian)斥(chi)責他(ta)說：“你是我的兒(er)子(zi)(zi)，擁有重兵，屢次被白虜小兒(er)摧敗，還活著(zhu)干什(shen)么！”苻(fu)暉(hui)憤(fen)懣惱(nao)怒(nu)而自殺。關中三(san)千(qian)余所堡壘(lei)，推舉平遠將(jiang)軍(jun)馮翊、趙敖為統領，相互結盟，派(pai)兵送糧援助苻(fu)堅(jian)。前秦左將(jiang)軍(jun)茍(gou)(gou)池(chi)、右(you)將(jiang)軍(jun)俱(ju)石子(zi)(zi)率騎(qi)兵五千(qian)，與(yu)慕(mu)容沖搶麥子(zi)(zi)，在驪山交戰，被慕(mu)容沖擊敗，茍(gou)(gou)池(chi)戰死，俱(ju)石子(zi)(zi)逃奔鄴城。苻(fu)堅(jian)大怒(nu)，又(you)派(pai)領軍(jun)將(jiang)軍(jun)楊定率身邊的精銳騎(qi)兵二千(qian)五百人(ren)攻擊慕(mu)容沖，大敗慕(mu)容沖，俘獲擄掠(lve)鮮卑人(ren)一萬多而回(hui)。苻(fu)堅(jian)發怒(nu)，把(ba)這些鮮卑人(ren)全部(bu)活埋。楊定勇猛善戰，慕(mu)容沖很害怕他(ta)，就(jiu)挖(wa)了陷馬坑以鞏固防地。
太元十年（385年）五月，慕容(rong)沖(chong)率(lv)軍登入(ru)長(chang)安城，苻堅(jian)身(shen)披鐘甲，督戰抵御，渾身(shen)中(zhong)箭，血(xue)流遍體。后被慕容(rong)沖(chong)擄掠的(de)(de)三(san)輔人士，都派使者告訴苻堅(jian)，請求放(fang)火(huo)作為(wei)內應(ying)。苻堅(jian)派七百騎(qi)兵去接應(ying)。可是在(zai)慕容(rong)沖(chong)營中(zhong)放(fang)火(huo)的(de)(de)人被風吹(chui)的(de)(de)火(huo)焰燒(shao)死(si)很多，只有(you)(you)十分之一二的(de)(de)人免于死(si)難。而(er)慕容(rong)沖(chong)在(zai)關中(zhong)殘暴肆虐，百姓都流亡失(shi)散，道(dao)路斷絕，方(fang)圓千里沒有(you)(you)人煙。
苻堅(jian)派(pai)衛將軍楊定在城西攻擊(ji)慕(mu)容(rong)(rong)沖(chong)，被慕(mu)容(rong)(rong)沖(chong)擒獲(huo)。苻堅(jian)更加恐(kong)懼，把后面(mian)的事交付給苻宏，帶著中(zhong)山(shan)(shan)公苻詵(shen)、張夫人率(lv)騎兵(bing)數百到五(wu)將山(shan)(shan)，遍告州郡，約(yue)定初冬救長(chang)安。苻宏不久就帶著母妻(qi)宗室男女幾千騎出(chu)奔，百官逃散。慕(mu)容(rong)(rong)沖(chong)進占長(chang)安，縱兵(bing)大肆擄掠，死者(zhe)不計其數。
慕(mu)容沖(chong)與(yu)苻堅相互攻戰時，軍勢(shi)很盛。后秦國主姚萇(chang)想要西上，擔心慕(mu)容沖(chong)阻遏他(ta)，就派使者與(yu)他(ta)通和，送兒子姚崇到慕(mu)容沖(chong)那里做人質(zhi)，進駐北地，練(lian)兵聚糧，以觀(guan)望時局的(de)變化。
太元(yuan)十年（385年）十月，慕容沖派車騎大將(jiang)軍高(gao)蓋(gai)率(lv)軍五(wu)萬進(jin)攻后(hou)秦，在新平以南交戰，大敗高(gao)蓋(gai)，高(gao)蓋(gai)率(lv)部下數千人來投(tou)降了(le)后(hou)秦。
太元十(shi)一年（386年），慕(mu)容(rong)(rong)沖喜(xi)歡住(zhu)在長安(an)，而且畏(wei)懼后(hou)燕武成帝慕(mu)容(rong)(rong)垂的強(qiang)盛，不敢東歸，便督促農耕，建筑宮室，作(zuo)長久(jiu)安(an)居(ju)的打(da)算。鮮卑(bei)人全都(dou)怨(yuan)恨(hen)他。左將(jiang)軍韓延順應眾人心(xin)中的不滿，攻打(da)慕(mu)容(rong)(rong)沖，并將(jiang)慕(mu)容(rong)(rong)沖殺(sha)死，擁立慕(mu)容(rong)(rong)沖的將(jiang)領段(duan)隨為西燕王，改(gai)年號為昌平。
太元九年（384年），高蓋、宿勤(qin)崇等殺慕容泓，立(li)慕容沖為皇(huang)太弟，秉(bing)承帝旨行(xing)事，自行(xing)設(she)置任命官員，任命高蓋為尚書令(ling)。后秦(qin)姚(yao)萇派(pai)遣兒子姚(yao)嵩到慕容沖處作為人質，以(yi)請求和好。
太元十(shi)年(nian)（385年(nian)），慕(mu)(mu)容沖稱(cheng)帝(di)(di)，改元更始。慕(mu)(mu)容沖稱(cheng)帝(di)(di)后躊躇(chu)滿志，任意(yi)賞罰(fa)官將。
太元十一年（386年），慕容沖(chong)喜歡(huan)居住在(zai)長安(an)，而且(qie)又畏懼慕容垂的(de)強盛，不敢東歸(gui)，便督(du)促農耕，建(jian)筑宮室，作長久安(an)居的(de)打算，因此遭致鮮卑人的(de)怨(yuan)恨(hen)，最終被(bei)殺(sha)。
巧笑倩兮 
