At day one at Leipzig Napoleon had the allied main force of Prussians, Russians and Austrians pressed very hard at Wachau, where the Zar also stayed. But the forces that could have carried through a decission were engaged simultaneously by Austrians from Lindenau and other Prussians from Möckern, and Schwarzenberg from his stand in the southern centre very swiftly sent his own reserve to Wachau were the Austrian Cuirassiers in the last minute saved the Prussians from being run over. Further east the Zars stand was close to being run over by French cavalry had the Zar not long last committed his own guard.
It is not difficult to imagine a few things going slightly different and Napoleon rolls up the allied at Wachau and takes the Zar and thousands of soldiers prisoner. With the allied main force defeated and split Napoleon would have a number of options:
1. Pursuit the remnants of the allied main army (probably in a SE direction)
2. Turn for home ASAP through Lindenau (where III. Austrian Armycorps is)
3. Engage the Austrians (II. AC + two reserve corps) between Elster and Pleisse (where Swarzenberg is)
4. Engage Blüchers Prussians at Möckern (to the North).
1. Would mean going further into enemy land and politically not much more could be reached now that the Zar is taken prisoner. But a couple of Armycorps will probably be detached in this direction.
2. It would not be difficult to push aside a single enemy Armycorps, but going home now would be like fleeing after a defeat – not an option for old Boney.
3. Schwarzenberg would be a great prize, but the terrain is difficult and give Scwarzenberg good opportunity to escape into the Erzgebirge, like he had done so many times before. But forces will have to be detached to watch him. Not many though as the terrain also makes it difficult for Schwarzenberg to attack from here.
4. Blücher is presently locked in a hard battle with Marmont, and a few armycorps extra, perhaps the old guard, would be enough to overrun Blücher. With Blücher defeated there still is Bernadotte with his army somewhere out there, but he will not be strong enough to stop the Grande Armee from going on Berlin and finishing off with the King of Prussia and his rabble-rousing Queen. This is what Napoleon will do (IMHO).
I guess Schwarzenberg will withdraw into Erzgebirge now, as that will mean the Austrian army leaving mainly intact (only IV. AC is out on a limp on the eastern flank) - that will be handy at the peace conference. I’m sure Bernadote when hearing the news will go home to Sweden in a hurry. Afterall he stayed in safe distance of the battle exactly as a precaution if this happened!
The Prussian King probably flees Berlin, but has no option but to show up at the peace conference at Versailles which the Austrian Foreign Secretary Metternich now says he always had thought was the best idea. Before the conference can be called Napoleon has an old business to deal with in Spain and arrives there with the Old Guard in record time. In a number of battles in quick succession he defeats Wellington decisively, who only narrowly escapes with the remnants of the British Army evacuated by the RN. With her fleet undefeated the British however refuse to show up at any peace conference.
That of course can’t stop Napoleon, and at the conference he makes a great show out of setting the Zar free and humilating him and not at least the King of Prussia. The King of Sweden (Bernadotte) never shows up, as he one morning is found with a bullet through his head.
In the new Europe that is created Austria under Napoleons father in law is left relatively unharmed compared to 1812 borders (Galicia to Poland-Lithuania), but Prussia and Russia must give up large territories to Saxony, Denmark and the new Polish-Lithuanian Kingdom (1930 Poland, Lithuania, Latvia, Belorus, parts of Ukraine). Sweden is given a proposal it can’t refuse to elect Frederik VI of Denmark-Norway as King (Frederik can’t refuse either). For ease his reign is called the Kingdom of North. Finland, Estonia and Ingermanland (incl. Sct. Petersburg) is gathered in the Grand Duchy of Finland. The Confederation of the Rhine is largely left unchanged.
The British of course keep blockading the continent, but it really is a good question who is blockading who. And the British have increasing difficulties in getting tar and timber for their fleet while navy yards are busy all over the French Empire and its vassals. Generals, Admirals and Kings all dream of being allowed a glorious role in the invasion that everybody expects.
In London it is realised that the last chance to create a new coalition has passed. The Europeans do not want war any longer and there is growing consensus about the British agitation and bribery being the real cause behind two decades of war.
In Paris Napoleon is assured that he will soon have a navy capable of defeating perfidious Albion, but doubt is in his mind. This is a kind of warfare he doesn’t understand, and wasn’t he also superior in 1805 – and did that help? A defeated invasion attempt would seriously distrub the consolidation process now happening in his Empire, so perhaps start negotioations...
The British are relieved by not having to look for invasion fleets any longer and basically gets away with giving back France the overseas pocessions lost since 1789 plus a few extra like Gibraltar and Quebec.
The next years are generally seen as the happy years as people are going back to their peacetime occupations. Now and then there is unrest somewhere, but there is allways a Prince or King nearby more than willing to serve the Empeor by crushing the rebellion. And then the Emperor allways can come by and do the comforting.
But in 1821 Napoleon become ill and dies. His son, the King of Rome, is of course crowned in a ceremony of a splendour not yet seen on the planet, but although a well meaning young man, his rule soon evolves into a chaotic fight between rivalling and corrupt fractions at the court, each one usually headed by one of his fathers old Fieldmarshals.
The Princes and Kings in Europe of couse can see this and the larger of them soon begin dreaming of one day becomming the Emperor...
Regards
Steffen Redbeard