太和五年(370年),前燕(yan)被(bei)前秦所滅(mie)亡后,包括慕(mu)(mu)容(rong)(rong)沖(chong)(chong)及其(qi)兄慕(mu)(mu)容(rong)(rong)泓在內的(de)眾多(duo)鮮卑慕(mu)(mu)容(rong)(rong)部人被(bei)遷往關(guan)中(zhong)(zhong)。太元九(jiu)年(384年),慕(mu)(mu)容(rong)(rong)沖(chong)(chong)之(zhi)叔慕(mu)(mu)容(rong)(rong)垂叛秦建立后燕(yan),慕(mu)(mu)容(rong)(rong)泓亦于關(guan)中(zhong)(zhong)舉兵自(zi)稱(cheng)濟北王。時任平陽太守的(de)慕(mu)(mu)容(rong)(rong)沖(chong)(chong)在河(he)東起兵,擁有(you)二萬(wan)兵眾,進軍(jun)攻(gong)打(da)蒲坂,苻(fu)堅令竇沖(chong)(chong)征討他。竇沖(chong)(chong)在河(he)東攻(gong)擊慕(mu)(mu)容(rong)(rong)沖(chong)(chong),大敗慕(mu)(mu)容(rong)(rong)沖(chong)(chong),慕(mu)(mu)容(rong)(rong)沖(chong)(chong)率騎兵八千投奔其(qi)兄慕(mu)(mu)容(rong)(rong)泓軍(jun)中(zhong)(zhong)。
不久慕容(rong)泓(hong)進(jin)軍(jun)長安,建立(li)西燕政權。太元九年(nian)(384年(nian))六(liu)月,慕容(rong)泓(hong)的謀(mou)臣高蓋、宿勤(qin)崇等認為(wei)慕容(rong)泓(hong)的德望不如慕容(rong)沖,而且執法(fa)苛嚴,于(yu)是(shi)就殺(sha)了慕容(rong)泓(hong),立(li)慕容(rong)沖為(wei)皇太弟,秉承帝旨行事,自行設置任(ren)命官員。 任(ren)命高蓋為(wei)尚(shang)書令。后秦姚(yao)萇派遣兒子姚(yao)嵩到慕容(rong)沖處作為(wei)人(ren)質(zhi),以請求和好。
太元九年(nian)(384年(nian))七月(yue),苻(fu)(fu)(fu)堅聽說(shuo)慕容沖(chong)離長(chang)安二百余里,就撤(che)兵(bing)而回,令撫(fu)軍(jun)將(jiang)軍(jun)苻(fu)(fu)(fu)方戍守驪(li)山,拜苻(fu)(fu)(fu)暉為(wei)(wei)使持節(jie)、散騎(qi)常(chang)侍、都(dou)督中外諸軍(jun)事、車騎(qi)大(da)將(jiang)軍(jun)、司隸校尉、錄尚書(shu)(shu),配給五萬(wan)兵(bing)力抵御慕容沖(chong),河間(jian)公苻(fu)(fu)(fu)琳為(wei)(wei)中軍(jun)大(da)將(jiang)軍(jun),作(zuo)為(wei)(wei)苻(fu)(fu)(fu)暉的后援(yuan)部隊。慕容沖(chong)就命婦人乘(cheng)牛馬充數(shu),舉竿(gan)子為(wei)(wei)旗(qi),揚土為(wei)(wei)灰塵,督促勉勵他的軍(jun)眾,早晨(chen)在鄭(zheng)西進攻苻(fu)(fu)(fu)暉軍(jun)營。苻(fu)(fu)(fu)暉出兵(bing)拒戰,慕容沖(chong)揚塵擊鼓(gu)吶(na)喊,苻(fu)(fu)(fu)暉軍(jun)戰敗。苻(fu)(fu)(fu)堅又任尚書(shu)(shu)姜宇為(wei)(wei)前將(jiang)軍(jun),與苻(fu)(fu)(fu)琳率軍(jun)三萬(wan),到灞(ba)上攻擊慕容沖(chong),被慕容沖(chong)打敗,姜宇戰死,苻(fu)(fu)(fu)琳身中流箭(jian),慕容沖(chong)于是(shi)占據并入(ru)住(zhu)阿房城。
太元九年(nian)(384年(nian))九月(yue),慕(mu)容沖(chong)進逼長(chang)安,苻堅(jian)(jian)登(deng)上(shang)城頭觀(guan)望(wang),嘆息(xi)道(dao):“這(zhe)些虜賊從哪里出(chu)來(lai)(lai)(lai)的(de)(de)(de)?這(zhe)么(me)強盛!”大聲斥(chi)責(ze)慕(mu)容沖(chong)說:“你(ni)們這(zhe)些群奴(nu)(nu)正好放牧牛(niu)羊,為什么(me)來(lai)(lai)(lai)送死!”慕(mu)容沖(chong)說:“奴(nu)(nu)就奴(nu)(nu)罷,已經(jing)厭倦了為奴(nu)(nu)之苦(ku),又(you)要把你(ni)取而代之。”苻堅(jian)(jian)派(pai)使者(zhe)送一(yi)領(ling)錦袍(pao)(pao)給(gei)慕(mu)容沖(chong),稱詔(zhao)旨(zhi)說:“古(gu)人交戰,使者(zhe)在中間往來(lai)(lai)(lai)。卿遠來(lai)(lai)(lai)諸事草創,能不(bu)勞(lao)苦(ku)嗎?現贈送一(yi)領(ling)錦袍(pao)(pao),以(yi)表(biao)明本心(xin)。朕對卿的(de)(de)(de)恩分怎么(me)樣(yang),而一(yi)夜之間出(chu)現這(zhe)樣(yang)的(de)(de)(de)變故!”慕(mu)容沖(chong)命詹事回答,亦稱“皇太弟有令”:“孤現在心(xin)在天下,豈能顧念一(yi)領(ling)錦袍(pao)(pao)的(de)(de)(de)小恩惠(hui)。如果能知道(dao)天命,就可以(yi)君臣束手,停止交戰,早點把皇帝送出(chu)來(lai)(lai)(lai),自(zi)然會寬赦(she)苻氏,以(yi)酬報舊好,終將不(bu)讓以(yi)往所做的(de)(de)(de)善事成為沒有可相提并論的(de)(de)(de)獨美”。苻堅(jian)(jian)大怒(nu)說:“我不(bu)聽從王猛、陽平公之言(yan),使得(de)白虜敢(gan)到(dao)這(zhe)樣(yang)的(de)(de)(de)地(di)步。”
太(tai)元十(shi)年(385年)正月,慕(mu)(mu)(mu)容(rong)(rong)沖(chong)(chong)在阿房城(cheng)稱帝,改年號為更始。苻(fu)(fu)堅(jian)與慕(mu)(mu)(mu)容(rong)(rong)沖(chong)(chong)交戰,各有(you)勝(sheng)負(fu)。苻(fu)(fu)堅(jian)曾(ceng)被(bei)(bei)慕(mu)(mu)(mu)容(rong)(rong)沖(chong)(chong)軍(jun)(jun)包圍,前(qian)秦殿中上將軍(jun)(jun)鄧(deng)邁、左中郎將鄧(deng)綏、尚(shang)書郎鄧(deng)瓊相互說(shuo):“我(wo)們(men)家世代蒙受榮耀(yao)寵(chong)貴,先君為國家建立了(le)大(da)功,不能不盡忠效節(jie),以繼承先君之志(zhi)。再(zai)說(shuo)不能為君主之難獻身者,不是大(da)丈夫。”于是與毛長樂(le)等(deng)蒙上獸皮(pi),揚(yang)戈奮勇(yong)攻擊(ji)(ji)慕(mu)(mu)(mu)容(rong)(rong)沖(chong)(chong)軍(jun)(jun),慕(mu)(mu)(mu)容(rong)(rong)沖(chong)(chong)軍(jun)(jun)潰敗(bai)。慕(mu)(mu)(mu)容(rong)(rong)沖(chong)(chong)又派(pai)他的(de)尚(shang)書令(ling)高(gao)蓋率軍(jun)(jun)夜襲長安,攻陷南(nan)門,進入南(nan)城(cheng)。前(qian)秦左將軍(jun)(jun)竇沖(chong)(chong)、前(qian)禁將軍(jun)(jun)李辯等(deng)擊(ji)(ji)敗(bai)高(gao)蓋軍(jun)(jun),高(gao)蓋軍(jun)(jun)眾被(bei)(bei)斬首一(yi)千(qian)八百人,并被(bei)(bei)前(qian)秦軍(jun)(jun)分尸而吃(chi)。接著苻(fu)(fu)堅(jian)又在城(cheng)西打敗(bai)慕(mu)(mu)(mu)容(rong)(rong)沖(chong)(chong),追擊(ji)(ji)慕(mu)(mu)(mu)容(rong)(rong)沖(chong)(chong)的(de)敗(bai)軍(jun)(jun)到阿房城(cheng)。前(qian)秦眾將請求(qiu)乘(cheng)勝(sheng)入城(cheng),苻(fu)(fu)堅(jian)擔心被(bei)(bei)慕(mu)(mu)(mu)容(rong)(rong)沖(chong)(chong)俘獲(huo),就嗚鑼罷兵。
太元十年(nian)(385年(nian))三(san)月,苻(fu)暉(hui)屢(lv)次被(bei)慕(mu)(mu)(mu)容沖(chong)(chong)(chong)(chong)(chong)擊(ji)(ji)敗,苻(fu)堅斥責他(ta)說:“你是(shi)我的兒子,擁有重兵,屢(lv)次被(bei)白(bai)虜小兒摧敗,還(huan)活著干什么!”苻(fu)暉(hui)憤懣惱怒(nu)而(er)自殺。關(guan)中(zhong)三(san)千余所(suo)堡壘,推舉平遠將(jiang)軍(jun)馮(feng)翊、趙(zhao)敖為(wei)統領,相互結盟,派兵送糧援助(zhu)苻(fu)堅。前秦左將(jiang)軍(jun)茍(gou)池、右將(jiang)軍(jun)俱(ju)石子率騎兵五(wu)千,與慕(mu)(mu)(mu)容沖(chong)(chong)(chong)(chong)(chong)搶(qiang)麥子,在(zai)驪山(shan)交戰(zhan),被(bei)慕(mu)(mu)(mu)容沖(chong)(chong)(chong)(chong)(chong)擊(ji)(ji)敗,茍(gou)池戰(zhan)死,俱(ju)石子逃奔鄴城。苻(fu)堅大怒(nu),又派領軍(jun)將(jiang)軍(jun)楊定率身(shen)邊的精銳(rui)騎兵二千五(wu)百人攻擊(ji)(ji)慕(mu)(mu)(mu)容沖(chong)(chong)(chong)(chong)(chong),大敗慕(mu)(mu)(mu)容沖(chong)(chong)(chong)(chong)(chong),俘(fu)獲擄掠鮮(xian)卑人一萬多(duo)而(er)回。苻(fu)堅發怒(nu),把這(zhe)些鮮(xian)卑人全部活埋。楊定勇猛善戰(zhan),慕(mu)(mu)(mu)容沖(chong)(chong)(chong)(chong)(chong)很害怕他(ta),就挖了陷馬(ma)坑以鞏(gong)固(gu)防地。
太(tai)元十年(nian)(385年(nian))五(wu)月,慕(mu)容沖(chong)率軍(jun)登入(ru)長安城,苻(fu)堅身披鐘甲,督(du)戰抵御,渾身中箭,血流遍體。后被(bei)慕(mu)容沖(chong)擄掠的三(san)輔(fu)人士,都派(pai)使者告訴苻(fu)堅,請求放火(huo)作(zuo)為內(nei)應。苻(fu)堅派(pai)七百騎(qi)兵(bing)去接應。可是在慕(mu)容沖(chong)營中放火(huo)的人被(bei)風吹的火(huo)焰燒死(si)很多,只(zhi)有十分之一二的人免于(yu)死(si)難。而慕(mu)容沖(chong)在關中殘(can)暴肆虐,百姓都流亡失散,道(dao)路(lu)斷絕,方圓千里(li)沒有人煙(yan)。
苻(fu)(fu)堅派衛將軍(jun)楊定在城西攻擊(ji)慕(mu)容(rong)(rong)沖,被慕(mu)容(rong)(rong)沖擒獲。苻(fu)(fu)堅更加恐懼,把后面的(de)事交付給苻(fu)(fu)宏(hong),帶(dai)著中(zhong)山公(gong)苻(fu)(fu)詵、張夫人(ren)率騎(qi)兵數百到五(wu)將山,遍告州(zhou)郡,約(yue)定初冬(dong)救長安。苻(fu)(fu)宏(hong)不久就帶(dai)著母妻宗(zong)室男女(nv)幾千騎(qi)出奔(ben),百官逃散(san)。慕(mu)容(rong)(rong)沖進占長安,縱(zong)兵大肆擄掠,死者不計其數。
慕容沖(chong)與(yu)(yu)苻堅(jian)相互攻戰時,軍(jun)勢很盛。后秦國主姚萇想要西(xi)上,擔(dan)心慕容沖(chong)阻(zu)遏(e)他,就派(pai)使(shi)者與(yu)(yu)他通和,送兒子姚崇(chong)到慕容沖(chong)那里做人(ren)質,進駐北地,練兵(bing)聚糧,以觀(guan)望時局的(de)變化。
太元十年(385年)十月,慕容沖派車騎大將軍高蓋率(lv)軍五萬進攻后秦,在新平以南交(jiao)戰,大敗高蓋,高蓋率(lv)部下數千人來投(tou)降了后秦。
太(tai)元十一年(nian)(386年(nian)),慕(mu)容沖喜歡住在(zai)長安(an),而且(qie)畏懼(ju)后燕(yan)武(wu)成(cheng)帝慕(mu)容垂的(de)(de)(de)強(qiang)盛,不敢東歸,便督促農(nong)耕(geng),建筑宮室,作(zuo)長久安(an)居的(de)(de)(de)打算。鮮(xian)卑人(ren)全都怨(yuan)恨他。左將軍(jun)韓延(yan)順應(ying)眾(zhong)人(ren)心中的(de)(de)(de)不滿,攻打慕(mu)容沖,并將慕(mu)容沖殺死,擁立慕(mu)容沖的(de)(de)(de)將領段隨為西燕(yan)王,改年(nian)號(hao)為昌平。
太元九年(384年),高蓋、宿勤崇(chong)等(deng)殺慕容泓,立(li)慕容沖(chong)為皇太弟,秉(bing)承帝旨(zhi)行事(shi),自(zi)行設置任命官員,任命高蓋為尚書令。后秦姚萇(chang)派遣兒子姚嵩到慕容沖(chong)處作為人質,以請(qing)求和好。
太元十年(nian)(385年(nian)),慕容沖(chong)(chong)稱(cheng)帝,改元更始。慕容沖(chong)(chong)稱(cheng)帝后(hou)躊(chou)躇(chu)滿志(zhi),任意賞罰官(guan)將(jiang)。
太(tai)元十一年(386年),慕容沖喜歡(huan)居住在長(chang)安(an),而且又畏(wei)懼慕容垂的強(qiang)盛,不敢東歸,便督促農耕,建筑(zhu)宮室,作(zuo)長(chang)久安(an)居的打(da)算,因此遭致鮮卑人的怨恨,最終被(bei)殺(sha)。