太(tai)和五年(nian)(370年(nian)),前燕(yan)被前秦所滅亡后(hou),包括慕(mu)容沖(chong)(chong)及(ji)其兄慕(mu)容泓在內的眾多鮮卑(bei)慕(mu)容部(bu)人被遷往關中。太(tai)元九年(nian)(384年(nian)),慕(mu)容沖(chong)(chong)之叔(shu)慕(mu)容垂叛秦建立后(hou)燕(yan),慕(mu)容泓亦于(yu)關中舉兵(bing)(bing)自稱濟北(bei)王。時任平陽太(tai)守的慕(mu)容沖(chong)(chong)在河(he)東起兵(bing)(bing),擁有二萬(wan)兵(bing)(bing)眾,進軍攻打(da)蒲坂(ban),苻堅令竇沖(chong)(chong)征討他。竇沖(chong)(chong)在河(he)東攻擊慕(mu)容沖(chong)(chong),大(da)敗慕(mu)容沖(chong)(chong),慕(mu)容沖(chong)(chong)率騎兵(bing)(bing)八千(qian)投奔(ben)其兄慕(mu)容泓軍中。
不久慕(mu)(mu)容(rong)泓進軍長安,建立西燕政權。太元(yuan)九年(384年)六月,慕(mu)(mu)容(rong)泓的謀臣高(gao)蓋、宿(su)勤(qin)崇等認(ren)為慕(mu)(mu)容(rong)泓的德(de)望不如慕(mu)(mu)容(rong)沖(chong),而且執法苛(ke)嚴(yan),于(yu)是就殺了慕(mu)(mu)容(rong)泓,立慕(mu)(mu)容(rong)沖(chong)為皇(huang)太弟(di),秉承帝旨行(xing)事,自行(xing)設(she)置任(ren)命官員。 任(ren)命高(gao)蓋為尚(shang)書(shu)令。后秦姚萇派遣(qian)兒(er)子姚嵩到慕(mu)(mu)容(rong)沖(chong)處作為人質,以請求和好。
太(tai)元九(jiu)年(384年)七月,苻(fu)堅聽(ting)說慕(mu)(mu)(mu)容(rong)沖(chong)離長安二百余(yu)里(li),就(jiu)撤兵(bing)(bing)而回,令撫軍將軍苻(fu)方戍(shu)守(shou)驪(li)山(shan),拜苻(fu)暉為(wei)使持節、散騎常侍、都督中(zhong)外諸軍事、車(che)騎大將軍、司隸校尉、錄尚書(shu),配給(gei)五萬(wan)兵(bing)(bing)力(li)抵御(yu)慕(mu)(mu)(mu)容(rong)沖(chong),河間公苻(fu)琳為(wei)中(zhong)軍大將軍,作為(wei)苻(fu)暉的后(hou)援(yuan)部(bu)隊。慕(mu)(mu)(mu)容(rong)沖(chong)就(jiu)命婦人乘牛(niu)馬充數,舉(ju)竿子為(wei)旗,揚土為(wei)灰塵(chen)(chen),督促勉勵(li)他的軍眾,早晨在鄭西進攻苻(fu)暉軍營。苻(fu)暉出兵(bing)(bing)拒戰,慕(mu)(mu)(mu)容(rong)沖(chong)揚塵(chen)(chen)擊鼓吶喊(han),苻(fu)暉軍戰敗(bai)。苻(fu)堅又任尚書(shu)姜(jiang)宇(yu)為(wei)前將軍,與苻(fu)琳率軍三萬(wan),到灞上攻擊慕(mu)(mu)(mu)容(rong)沖(chong),被慕(mu)(mu)(mu)容(rong)沖(chong)打敗(bai),姜(jiang)宇(yu)戰死,苻(fu)琳身中(zhong)流(liu)箭,慕(mu)(mu)(mu)容(rong)沖(chong)于是占據并入(ru)住阿房城。
太元九年(384年)九月,慕(mu)(mu)容(rong)沖(chong)(chong)(chong)進逼長安,苻(fu)堅(jian)登上城頭觀(guan)望,嘆息道:“這(zhe)(zhe)些(xie)虜(lu)賊從哪里出(chu)(chu)來(lai)(lai)(lai)的(de)(de)?這(zhe)(zhe)么強盛!”大聲斥責慕(mu)(mu)容(rong)沖(chong)(chong)(chong)說:“你(ni)(ni)們這(zhe)(zhe)些(xie)群奴(nu)正(zheng)好放牧牛羊,為什么來(lai)(lai)(lai)送(song)死(si)!”慕(mu)(mu)容(rong)沖(chong)(chong)(chong)說:“奴(nu)就奴(nu)罷,已(yi)經厭(yan)倦(juan)了(le)為奴(nu)之苦(ku),又要把你(ni)(ni)取而代之。”苻(fu)堅(jian)派使(shi)(shi)者送(song)一(yi)領錦袍(pao)給慕(mu)(mu)容(rong)沖(chong)(chong)(chong),稱詔旨說:“古人交戰,使(shi)(shi)者在(zai)中間往來(lai)(lai)(lai)。卿(qing)遠來(lai)(lai)(lai)諸事草創,能(neng)不勞苦(ku)嗎?現贈送(song)一(yi)領錦袍(pao),以表明本心。朕對卿(qing)的(de)(de)恩分怎(zen)么樣,而一(yi)夜之間出(chu)(chu)現這(zhe)(zhe)樣的(de)(de)變故(gu)!”慕(mu)(mu)容(rong)沖(chong)(chong)(chong)命(ming)(ming)詹(zhan)事回(hui)答,亦稱“皇太弟有令(ling)”:“孤(gu)現在(zai)心在(zai)天下,豈能(neng)顧(gu)念(nian)一(yi)領錦袍(pao)的(de)(de)小恩惠(hui)。如果能(neng)知道天命(ming)(ming),就可以君(jun)臣束手,停止交戰,早點把皇帝送(song)出(chu)(chu)來(lai)(lai)(lai),自然會寬赦苻(fu)氏(shi),以酬報舊好,終將不讓以往所做的(de)(de)善事成為沒有可相提并論的(de)(de)獨美(mei)”。苻(fu)堅(jian)大怒說:“我不聽從王猛(meng)、陽平公之言(yan),使(shi)(shi)得白虜(lu)敢(gan)到這(zhe)(zhe)樣的(de)(de)地(di)步。”
太元十(shi)年(385年)正月,慕(mu)(mu)容(rong)(rong)(rong)(rong)沖在(zai)阿房城(cheng)稱帝,改年號(hao)為(wei)(wei)更(geng)始(shi)。苻(fu)(fu)堅(jian)與(yu)慕(mu)(mu)容(rong)(rong)(rong)(rong)沖交戰,各有勝負。苻(fu)(fu)堅(jian)曾(ceng)被慕(mu)(mu)容(rong)(rong)(rong)(rong)沖軍包(bao)圍(wei),前(qian)(qian)秦(qin)(qin)殿中(zhong)上(shang)將(jiang)(jiang)軍鄧(deng)邁(mai)、左(zuo)中(zhong)郎將(jiang)(jiang)鄧(deng)綏、尚(shang)書(shu)郎鄧(deng)瓊相互說:“我們家世代(dai)蒙受榮耀寵(chong)貴(gui),先(xian)君為(wei)(wei)國(guo)家建立了大功,不(bu)能不(bu)盡(jin)忠效節(jie),以繼承先(xian)君之志。再(zai)說不(bu)能為(wei)(wei)君主之難獻(xian)身者,不(bu)是大丈夫。”于是與(yu)毛長(chang)(chang)樂等(deng)蒙上(shang)獸皮,揚戈奮勇攻(gong)擊(ji)(ji)慕(mu)(mu)容(rong)(rong)(rong)(rong)沖軍,慕(mu)(mu)容(rong)(rong)(rong)(rong)沖軍潰敗。慕(mu)(mu)容(rong)(rong)(rong)(rong)沖又(you)派他的(de)尚(shang)書(shu)令高蓋(gai)率軍夜襲長(chang)(chang)安(an),攻(gong)陷南門(men),進入南城(cheng)。前(qian)(qian)秦(qin)(qin)左(zuo)將(jiang)(jiang)軍竇沖、前(qian)(qian)禁將(jiang)(jiang)軍李辯(bian)等(deng)擊(ji)(ji)敗高蓋(gai)軍,高蓋(gai)軍眾被斬首一千八百人,并被前(qian)(qian)秦(qin)(qin)軍分(fen)尸而(er)吃。接著(zhu)苻(fu)(fu)堅(jian)又(you)在(zai)城(cheng)西打敗慕(mu)(mu)容(rong)(rong)(rong)(rong)沖,追擊(ji)(ji)慕(mu)(mu)容(rong)(rong)(rong)(rong)沖的(de)敗軍到阿房城(cheng)。前(qian)(qian)秦(qin)(qin)眾將(jiang)(jiang)請求乘(cheng)勝入城(cheng),苻(fu)(fu)堅(jian)擔心被慕(mu)(mu)容(rong)(rong)(rong)(rong)沖俘獲(huo),就嗚鑼罷(ba)兵(bing)。
太元十年(nian)(385年(nian))三月,苻(fu)暉屢(lv)次(ci)被慕(mu)(mu)容(rong)(rong)(rong)沖擊(ji)敗(bai)(bai),苻(fu)堅(jian)斥責他說(shuo):“你是我的(de)兒子(zi),擁有重兵,屢(lv)次(ci)被白虜小兒摧敗(bai)(bai),還活著(zhu)干什么!”苻(fu)暉憤懣惱怒而自殺。關中三千余所(suo)堡壘,推舉平(ping)遠將(jiang)軍(jun)馮翊(yi)、趙(zhao)敖(ao)為(wei)統領,相互結盟(meng),派(pai)兵送(song)糧援(yuan)助苻(fu)堅(jian)。前秦(qin)左將(jiang)軍(jun)茍池(chi)、右將(jiang)軍(jun)俱石子(zi)率騎(qi)兵五千,與慕(mu)(mu)容(rong)(rong)(rong)沖搶(qiang)麥子(zi),在驪山交戰(zhan)(zhan),被慕(mu)(mu)容(rong)(rong)(rong)沖擊(ji)敗(bai)(bai),茍池(chi)戰(zhan)(zhan)死,俱石子(zi)逃奔鄴(ye)城。苻(fu)堅(jian)大怒,又派(pai)領軍(jun)將(jiang)軍(jun)楊(yang)定率身邊的(de)精銳(rui)騎(qi)兵二千五百人攻擊(ji)慕(mu)(mu)容(rong)(rong)(rong)沖,大敗(bai)(bai)慕(mu)(mu)容(rong)(rong)(rong)沖,俘獲擄掠鮮(xian)卑(bei)人一萬多(duo)而回。苻(fu)堅(jian)發怒,把這些鮮(xian)卑(bei)人全部活埋。楊(yang)定勇(yong)猛善戰(zhan)(zhan),慕(mu)(mu)容(rong)(rong)(rong)沖很害怕他,就挖了陷馬坑以鞏固防地(di)。
太元十年(nian)(385年(nian))五月,慕(mu)容(rong)沖(chong)率軍登入長安城,苻(fu)堅(jian)身披鐘(zhong)甲(jia),督戰抵御,渾(hun)身中(zhong)箭(jian),血(xue)流(liu)遍體。后被慕(mu)容(rong)沖(chong)擄掠的三輔人士,都派(pai)使者告訴(su)苻(fu)堅(jian),請求(qiu)放(fang)火(huo)作(zuo)為(wei)內應。苻(fu)堅(jian)派(pai)七百騎兵去(qu)接應。可(ke)是在(zai)慕(mu)容(rong)沖(chong)營(ying)中(zhong)放(fang)火(huo)的人被風(feng)吹的火(huo)焰(yan)燒死很多,只有十分之一二的人免于死難。而慕(mu)容(rong)沖(chong)在(zai)關(guan)中(zhong)殘(can)暴肆(si)虐(nve),百姓都流(liu)亡失(shi)散,道路斷絕,方圓(yuan)千里沒有人煙。
苻(fu)堅(jian)派衛(wei)將軍楊定(ding)在(zai)城(cheng)西攻擊慕容(rong)沖,被(bei)慕容(rong)沖擒獲(huo)。苻(fu)堅(jian)更加恐(kong)懼,把后面的事交付給(gei)苻(fu)宏,帶(dai)著中山(shan)(shan)公苻(fu)詵、張夫人率騎(qi)兵(bing)數(shu)(shu)百到五將山(shan)(shan),遍告州郡,約定(ding)初(chu)冬(dong)救長安(an)。苻(fu)宏不(bu)久就帶(dai)著母妻宗室男女(nv)幾(ji)千騎(qi)出奔,百官(guan)逃散。慕容(rong)沖進占長安(an),縱兵(bing)大(da)肆(si)擄掠,死者(zhe)不(bu)計其數(shu)(shu)。
慕(mu)容(rong)沖與(yu)苻堅相互攻戰時,軍勢很盛。后秦國(guo)主姚萇想要(yao)西(xi)上,擔心慕(mu)容(rong)沖阻遏(e)他(ta),就派使者與(yu)他(ta)通(tong)和,送兒子(zi)姚崇到慕(mu)容(rong)沖那里(li)做(zuo)人質,進(jin)駐北地,練兵聚糧(liang),以觀望時局的變化(hua)。
太元十年(nian)(nian)(385年(nian)(nian))十月(yue),慕(mu)容沖派車騎大(da)將軍高(gao)(gao)蓋(gai)率軍五萬進(jin)攻后秦,在新平以南交(jiao)戰,大(da)敗高(gao)(gao)蓋(gai),高(gao)(gao)蓋(gai)率部下數千人來投降了(le)后秦。
太元十一年(nian)(386年(nian)),慕(mu)(mu)容(rong)沖喜歡住在長(chang)(chang)安,而且(qie)畏懼后燕(yan)武成帝(di)慕(mu)(mu)容(rong)垂的(de)(de)強盛,不敢東歸(gui),便督促農耕,建筑宮(gong)室,作長(chang)(chang)久安居的(de)(de)打(da)算。鮮卑人(ren)全都怨(yuan)恨他。左將(jiang)軍韓延順應眾人(ren)心中的(de)(de)不滿,攻(gong)打(da)慕(mu)(mu)容(rong)沖,并將(jiang)慕(mu)(mu)容(rong)沖殺死,擁立慕(mu)(mu)容(rong)沖的(de)(de)將(jiang)領段(duan)隨為(wei)(wei)西燕(yan)王,改年(nian)號為(wei)(wei)昌平。
太元九年(384年),高蓋、宿勤崇(chong)等殺慕容泓,立慕容沖為(wei)皇(huang)太弟,秉承帝旨行事,自行設置任命官員,任命高蓋為(wei)尚書令。后秦姚(yao)萇(chang)派遣兒子(zi)姚(yao)嵩到慕容沖處作為(wei)人(ren)質,以請求和好(hao)。
太(tai)元(yuan)十年(385年),慕容(rong)沖稱(cheng)帝(di),改元(yuan)更始。慕容(rong)沖稱(cheng)帝(di)后(hou)躊躇滿(man)志,任(ren)意賞罰官(guan)將。
太元十一(yi)年(nian)(386年(nian)),慕容沖喜歡居住(zhu)在長安,而(er)且又畏懼(ju)慕容垂的強盛,不敢東歸(gui),便督(du)促農耕,建筑宮室,作長久安居的打算,因此(ci)遭致鮮卑(bei)人的怨恨,最終被殺。