百日王朝 (法语：Cent-Jours) 是指拿破仑一世在被流放后重返法国，企图重建法兰西第一帝国的一连串事件。
1815年3月20日，拿破仑从厄爾巴島逃跑到法国，集结军队，把刚复辟的波旁王朝推翻，再度称帝；6月28日，因为滑鐵盧戰役的失败，拿破仑再被流放到圣赫勒拿岛，波旁王朝再度复辟。拿破仑战争至此结束。
事件也被称为第七次反法联盟之战。联盟包括英国、俄罗斯、普鲁士、瑞典、奥地利与数个德意志邦国。她们都宣称拿破仑是罪犯，而不是法国皇帝。
流放的日子
拿破仑以很特别的方法东山再起。1815年2月26日，当英法警卫舰队不在时，拿破仑与大约600名追随者逃离意大利Portoferraio，于3月1日到达法国南岸城市Antibes。除了有较多保皇势力的普罗旺斯之外，所有法国地区的人民都热烈欢迎拿破仑。他们对波旁王室毫无好感，所以拿破仑不废多少工夫，就重夺权力。他马上重建势力：3月5日，第五与第七步兵师投靠拿破仑。曾经投靠波旁王朝的米歇尔·内伊也倒戈相向，在3月14日带领六千名士兵向拿破仑投诚；五日之后，在路易十八逃走后，拿破仑与他的军队进驻巴黎。
曾经有一则趣闻轶事显示拿破仑的魅力与声望：波旁皇帝派出士兵，务求阻止拿破仑夺权。双方对峙，准备开枪。正要开战时，拿破仑走出来，站在两军之间，面对着对方士兵。他扯开自己的大衣，高呼：“如果你们想开枪射向你们的皇帝，快射吧！”。不过，后世研究指出，当时拿破仑知道对方的枪支盛着的只是粉末而已。
拿破仑并没有被人民的热情冲昏头脑，深深知道权力得来不易。全凭人民支持他、讨厌患痛风的老国王以及其贪婪的朝臣对他谄媚，他才可以和平地夺取政权。他也知道法国人民一定不会容忍独裁，所以一定要想办法好好管理新法国。于是，他在返回巴黎途中，不断宣传自己会实现改革与以宪法治国。拿破仑政府提出不少承诺，既满足人民的期望，也尝试消除邻国的忧虑与疑心。
重返法国
考虑到拿破仑的心力与健康，王朝能复兴与否也许值得商榷。但一般认为，拿破仑看似不济，但仍然有捲土重来的能力。基于局势已变，拿破仑不禁有感困惑，好像能解释事实；他左右为难，面对臣下与民众希望建立君主立宪的要求，似乎变得有心无力。而且，他还要与从前对其绝对服从的部下妥协，故此身心俱疲。不过，当他放下议会事务，重返战场的时候，拿破仑又重新显示其军事天才与领导才能，再度缔造他在1814年的辉煌战绩。
有人认为拿破仑政权之衰落，始于俄法战争之失败。但另有一说指出这种论调不正确：令拿破仑颓废的流放生活，比俄罗斯之冬的杀伤力更大。在厄尔巴岛时，他变得毫无生气，又变得愈来愈胖；在1815年初，他更开始有肠脏问题，虽然不算很严重。总的来说，法国的转变比拿破仑的健康更重要，又大大限制了这位法国皇帝的权力。他曾经对本杰明·贡斯当 (Benjamin Constant) 说过：“我老了。也许，当一个安静的立宪皇帝比较适合我，更适合我的儿子。”这一说法认为，拿破仑清楚明白当下的情况，但心里始终不接受立宪，故此一直心绪不安。
再度掌权
The attempts of the royalists gave him little concern: the duc d'Angoulême raised a small force for Louis XVIII in the south, but at Valence it melted away in front of Grouchy's command; and the duke, on 9 April 1815, signed a convention whereby they received a free pardon from the emperor. The royalists of la Vendée moved later and caused more trouble. But the chief problem centred in the constitution. At Lyon, on 13 March 1815, Napoleon had issued an edict dissolving the existing chambers and ordering the convocation of a national mass meeting, or Champ de Mai, for the purpose of modifying the constitution of the Napoleonic empire. That work was carried out by Benjamin Constant in concert with the emperor. The resulting Acte additionel (supplementary to the constitutions of the empire) bestowed on France an hereditary chamber of peers and a chamber of representatives elected by the "electoral colleges" of the empire, which comprised scarcely one hundredth part of the citizens of France. As Châteaubriand remarked, in reference to Louis XVIII's constitutional charter, the new constitution — La Benjamine, it was dubbed — was merely a slightly improved charter. Its incompleteness displeased the liberals; it garnered only 1,532,527 votes in the plebiscite, a total less than half of those of the plebiscites of the Consulate.
Not all the gorgeous display of the Champ de Mai (held on 1 June 1815) could hide the discontent at the meagre fulfilment of the promises given at Lyon. Napoleon ended his speech with the words "My will is that of the people: My rights are its rights." The words rang hollow as was seen when, on 3 June, the deputies chose, as president of their chamber, Lanjuinais, the staunch liberal who had so often opposed the emperor. The latter was with difficulty dissuaded from quashing the election.
Other causes of offence arose, and Napoleon in his last communication to them warned them not to imitate the Greeks of the later Empire, who engaged in subtle discussions when the ram was battering at their gates. On the next day (12 June 1815) he set out for the northern frontier. His spirits rose at the prospect of rejoining the army. At Saint Helena he told Gourgaud that he intended in 1815 to dissolve the chambers as soon as he had won a great victory.
Discontent
In point of fact, the sword alone could decide his fate, both in internal and international affairs. Neither France nor Europe took seriously his rather vague declaration of his contentment with the role of constitutional monarch of France. No one believed that he would be content with the "ancient limits". So often had he declared that the Rhine and the Netherlands were necessary to France that everyone looked on his present assertions as a mere device to gain time. As far back as 13 March, six days before he reached Paris, the powers at the Congress of Vienna declared him an outlaw; four days later the United Kingdom, Russia, Austria and Prussia bound themselves to put 150,000 men into the field to end his rule. Their recollection of his conduct during the congress of Châtillon was the determining fact at this crisis; his professions at Lyon or Paris had not the slightest effect; his efforts to detach Austria from the coalition, as also the feelers put forth tentatively by Fouché at Vienna, were fruitless. The coalitions, once so brittle as to break at the first strain, had now been hammered into solidity by his blows. If ever a man was condemned by his past, Napoleon was so in 1815.
Napoleon knew that, once his attempts at dissuading one or more of the allies from invading France had failed, his only chance of remaining in power was to attack before the Allies put together an overwhelming force. If he could destroy the existing Allied forces in Belgium before they were reinforced, he might be able to drive the British back to the sea and knock the Prussians out of the war. This was a successful strategy he had used many times before.
Napoleon moved two armies, the Army of the North (AotN) and the Reserve Army (RA), up to the French Belgium frontier without alerting the Allies. He crossed the frontier and split his AoTN in two. He took the RA and the right wing of the AotN and attacked the Prussians under the command of General Gebhard Leberecht von Blücher at the Battle of Ligny on June 16 1815. The left wing of the army under Marshal Ney proceeded to block the Nivelles-Namur road at the crossroads of Quatre Bras so that the British-allied forces under the Duke of Wellington could not go to the aid of the Prussians. Ney's wing of the French army engaged Wellington's forces in the Battle of Quatre Bras on the same day as Napoleon engaged the Prussians. The outcome of the day of fighting was that, at Quatre Bras, Ney stopped any of Wellington's forces going to the aid of Blücher's Prussians and Napoleon, although unable to destroy the Prussian army, forced it to retreat in disarray.
On the morning of the June 17 Napoleon sent the right wing of the Army of the North under the command of Marshal Grouchy to harass the Prussians to stop them reforming. He set off via Quatre Bras with the RA and combined his forces with the left wing of the AotN to pursue Wellington's forces, which were retreating towards Brussels. Just before the small village of Waterloo, Wellington deployed most of his forces on the rear side of an escarpment. He placed some of his forces in front of the main deployment in two fortified farmhouses at the base of the escarpment, which guarded the two roads to Brussels. It was here on June 18 1815 that the decisive European battle of the 19th century took place. Within the sound of cannon fire a second battle took place at the village of Wavre. Grouchy, who was dilatory in his pursuit of the Prussians, failing to stop them regrouping after their defeat at Ligny, attacked the Prussian III Corps under the command of General Johann von Thielmann, believing that he was engaging the rearguard of a still-retreating Prussian force. However only one Corps remained — the other three Prussian Corps (I, II and the still fresh IV) were marching towards Waterloo.
战火重燃主条目：滑铁卢战役
由于昨晚下过雨，拿破仑等待地面变干，延迟了数小时才于6月18日在滑铁卢开战。到了近黄昏为止，法军仍未能击退陡波上威灵顿的军队。当普鲁士大军压境兵攻击法军右翼时，拿破仑企图阻止联军会合的策略失败了。法军败走，而联军则步步紧迫。6月19日早上，Grouchy将军带领法军赢得Wavre之战役。他成功带领北方军队撤退，让法军重整旗鼓，但为时已晚，皆因法军大势已去，不能抵抗联军猛攻，只能撤退到巴黎。
滑鐵盧戰役
滑铁卢战役结束三天后，拿破仑返回巴黎，但仍然希望平息国内的不满。然而，议会与民众不容许拿破仑继续执政。只有拿破仑与他的弟弟Lucien Bonaparte仍然相信他们可以解散议会和建立独裁，以期力挽狂澜于既倒。不过，就连军部总长Louis Nicolas Davout将军也认为，法国的命运之在乎议会的决定。
拿破仑的成功，曾经吸引法国人民远离她从1789年起坚持的原则，但现在法国又重返起点。拿破仑从1796年到1814年的战役令法国大大扩张版图；但现在的失败却可能令法国得不偿失。与其负隅顽抗，不如由Charles Maurice de Talleyrand-Périgord以“正统原则”捍卫法国。拿破仑最后认清事实，终于在6月22日退位予其子拿破仑二世。由于儿子仍在奥地利，所以传位仅仅具有象征性。6月25日，新临时政府总理Fouché暗地里向拿破仑通知，告诉他一定要逃离巴黎。拿破仑到达前妻约瑟芬·博阿尔内的家Chateau de Malmaison，亦即约瑟芬在他第一次退位前逝世的地方。6月29日，普鲁士军队继续追击，要抓到拿破仑，无论是生是死。拿破仑于是向西逃到Rochefort, Charente-Maritime，希望能逃到美国。
这位前法国皇帝在7月17日被捕，最后再度被驱逐。路易十八则返回巴黎，在联军保护下在7月8日再度复辟。法兰西第一帝国正式结束。
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