Latest Point for the Holy Roman Empire to centralize?

The old cliche about the Holy Roman Empire is that it was neither Holy, Roman, nor an Empire. As other European nations like England and France slowly developed relatively strong national governments throughout, the HRE progressively fragmented in that period, with more power going to local nobility. Arguably, the Empire had ceased to exist as a meaningful political institution long before Napoleon made it official.

So, how could that have been prevented? What's the latest point in history that a Holy Roman Empire consisting, at minimum, of most of German-speaking Europe, could have turned around the process of political fragmentation and developed into a strong nation-state in the same way that the United Kingdom and France did?
 
What's the latest point in history that a Holy Roman Empire consisting, at minimum, of most of German-speaking Europe, could have turned around the process of political fragmentation and developed into a strong nation-state in the same way that the United Kingdom and France did?

I would say it's during the Thirty Years' War, more precisely during the Peace of Prague of 1635. The parties of the threaty were the Emperor Ferdinand II and the Elector of Saxony John George I of Saxony.

John George was a moderate, interested in maintaining the Peace of Augsburg of 1555. John George, who was a protestant, advocated for a policy of conciliation, supported the Habsburg emperors and opposed non-Lutheran Protestants, especially Calvinists. John George only allied with the Swedish forces under Gustav Adolph when troops of the Catholic Leage ravaged Saxony (to prevent a Swedish-Saxon alliance), and Saxon troops fought with Swedish ones at Breitenfeld in 1631, resulting in the liberation of Saxony from Catholic occupation. However, when Gustav Adolph died at Lützen in 1632 and the Swedish were beaten at Nördlingen in 1634, John Goerge started to seek for peace with the Imperial forces to preserve his Electorate. He refused to enter the Protestant Heilbronn League, lead by Sweden and influenced by France, and entered negotiations with the Holy Roman Emperor.

On the other side, Ferdinand II had just got rid of Wallenstein, who was hated by the Imperial Estates and thus a constant obstacle to peace. Ferdinand had, due to the Swedish invasion, lost all his conquests of the 1620s and knew that the moment for compromise with the Protestants had come. In the same time, the Swedish presence in South Germany crumbled after Nördlingen and the Germany was occupied by imperial troops. In this position of strength, Ferdinand could impose favourable terms, which at the same time were acceptale for the different princes. The central terms of the peace were:
  • The revocation of the Edict of Restitution. This imperial act of 1629 (when the imperial power was on its highest, just before the Swedish intervention) provided that Catholic ecclesiastical land that had been converted to Protestantism would have to return under Catholic rule. This edict had triggered a lot of resistance, even by Catholic princes who feared that imperial power might become to important.
  • The prohibition of alliances between princes or between princes and foreign powers. I think this is the most important therm of the peace treaty, since in the later history of the HRE, alliances between princes and foreign powers (like between Prussia and Great Britain during the Seven Years War) proved to be detrimental for the Empire's unity. Consequently, both the Catholic League and the Heilbronn Leaue were dissolved according with the treaty.
  • The formation of an imperial army. The various armies of the different Imperial Estates should the united into an army of the Holy Roman Empire unter the command of the emperor. The princes would be allowed to command their own troops, but only as generals of the emperor. The purpose of this term was to fight against foreign troops within the empire - the princes agreed to an unified army to expel the looting Swedish (and soon French) armies from their lands.
  • To win over the last opposing princes, an amnesty was granted all of those who fought against imperial troops, except the descendants of the Elector Palatine Frederick V.
This peace treaty was accepted by most princes, with the exceptio nof Bernard of Saxe-Weimar, who sided with the French, and William V of Hesse-Kassel, an ally of the Swedish. Another center of resistance was the Palatine, which was defeated only in 1638 in the Battle of Vlotho. While the treaty succeeded in dissolving the different leagues, it couldn't restore peace within the empire. Against all odds, the Swedish forces achieved a moralic victory at Dömitz in 1635, which ended the military ambitions of Saxony. A decisive Swedish victory in Battle of Wittstock in 1636 reversed the results of Nördlingen and started a phase of French and Swedish victory against the Imperialists, leading to the Westphalian Peace and the revocation of the Peace of Prague. One of the major term of the Westphalian Peace was that it wasn't the Empire, but the different estates that negociated with France and Sweden - thus, they gained the ius belli ac pacis, the right to declare war and conclude peace. In an alternate timeline where the Holy Roman Empire becomes a centralized state, this must not happen. The Emperor has to stay the master of the Empire's foreign policy.

If the Imperialists (including the Protestant estates, especially Saxony and Brandenburg) had beaten Sweden and France after 1635, the Peace of Prague could have led to a more united empire. Ferdinand III, who became emperor in 1637, was opposed to the Jesuits. I suppose he was a more moderate Catholic than is father and would have tried to find a compromise with the Protestant estates. Maybe the Peace of Prague would have lasted - and a united army and foreign policy is a good start for the Holy Roman Empire.
 
Depends on what your definition of centralization is. The last chance of anything similar to France would be the Thirty years war, but the last chance of a Federated Empire similar to that of the German Empire would be the Austrian Succession war. Before the Austrian succession war the Empire was more or less unified under the Habsburg dynasty; Prussia and Friedrich the Great's betrayal of Maria Theresa was quite shocking and permanently divided the Empire between Austria and Prussia. The rest of the century up to the Revolutionary wars was dominated by the rivalry between the two great powers. So having Austria beat Prussia means no divided Germany and ensures an eventual unification/centralization under the Habsburgs.
 
I don`t think the Thirty Years War is realistic. Not impossible, but low probabillity. Too powerful Habsburgs in the context of a confessionally divided country, especially when there`s already been such a big war.

I think the best chance is a different 15th century with regards to marital alliances and inheritances, and when the Protestant Reformation comes around, all German-speaking "Reichsglieder" forming their own, all-Protestant Empire, abandoning the Pope and Northern Italy to French and (non-Habsburg) Spanish (and Ottoman?) vultures.
 
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