The 1814 campaign was an extension of the 1813 campaign. In June 1813 after Dresden, Austria had offered Napoleon a moderate peace that was repeated in Frankfort in November after Leipzig. Wikipedia:

The proposal was that Napoleon would remain as Emperor of France, but France would be reduced to what the French revolutionaries claimed as France’s “Natural borders.” The natural borders were the Pyrenees mountains, the Alps mountains, and the Rhine River. France would retain control of Belgium, Savoy and the Rhineland (the west bank of the Rhine River), conquered and annexed during the early wars of the French Revolution, while giving up other conquests, including all of Spain, Poland and the Netherlands, and most of Italy and Germany east of the Rhine.

(…) Metternich told Napoleon these were the best terms the Allies were likely to offer; after further victories, the terms would become harsher and harsher. Metternich’s motivation was to maintain France as a balance against Russian threats, while ending the highly destabilizing series of wars. Napoleon, expecting to win the war, delayed too long and lost this opportunity.

In Spring 1814 the now cornered Napoleon tried to get the Frankfort proposal back on the table again but by then Czar Alexander had already decided to take Paris. The sudden victories in February caused a certain Allied confusion, but Alexander – according to historian Houssaye – had his own secret plan, and by the way, so had Napoleon:

on January 29, the Czar gave way to the representatives of Castlereagh and Metternich and agreed to open negotiations, but on the basis that France should revert to the frontiers she possessed in 1789, which was very different to the terms offered at Frankfort.

The Russian diplomat Razumowsky, received from the Czar secret instructions to drag out the negotiations as much as possible. While Napoleon offered

to negotiate on terms which he knew would be refused by the allies, who in their turn were only willing to treat on terms which the French could not accept. It was a comedy on both sides, planned and carried out with the sole object of deceiving public opinion.

So after the start of the negotiation in February the Russian diplomat immediately

objected that his credentials were not in order, and the conference was put off till the following day, but was again postponed for a day on account of Razumowsky’s credentials.

The French diplomat Caulaincourt could have accepted the Allied terms and end the campaign at once, but Napoleon had sent him contradictory orders: carte blanche and agree if acceptable “but if not, we will take the chances of a battle and run the risk of losing Paris and all that that will involve.” [He] often had no information except false news told him by the allies, who caused the diplomatic messages to be delayed to such an extent that couriers took six days to go and return from Napoleon’s head-quarters.

After the Montmirail victory, Napoleon revoked the carte blanche and ordered that only the Frankfort proposal was acceptable. He almost convinced Austria to accept an armistice. Caulaincourt made a sly proposal for an armistice: France was ready to give up fortresses near the Rhine border with Germany. Smart, because the armistice would

not have definitely pledged Napoleon’s word as to the territories to be handed over, and by giving up some fortresses on the left bank of the Rhine Napoleon would have regained the use of a veteran army. Finally, if the conditions should prove unacceptable, Napoleon would be free to denounce the armistice, and take the field with a larger and better-organized army.

The British foreign secretary discussed negotiations with the Czar, The Czar formally agreed, but in secret “Alexander sent fresh instructions to Razumowsky, bidding him to continue causing as much delay as possible.”

The Coalition recovered and Allied reinforcements approached the French border. Castlereagh heard about Napoleon’s secret ouvertures to Austria.

Lord Castlereagh(…) was afraid that another effort in the same direction might succeed in detaching Austria from the coalition (…) This treaty, which was the origin of the “Holy Alliance,” was signed on March 1 at Chaumont,(…) The contracting Powers bound themselves for a period of twenty years, during which they each undertook not to treat individually with France; England guaranteed for the whole period of hostilities an annual subsidy of 150,000,000 francs, to be divided between Russia, Austria, and Prussia, and each of these powers undertook to carry on the war with a contingent of 150,000 men.

That sealed Napoleon’s fate.

Next: Soissons, and how a Brigadier-General betrayed Bonaparte and sold the empire

main source: Houssaye, Henry. Napoleon and the Campaign of 1814 . Kindle Edition.