The Myth of Consensual Marriage
Friedrich II, King of Prussia, Elector of Brandenburg: I consent
Maria Theresia, Archduchess of Austria, Queen of Bohemia, Hungary, and Croatia: I consent
France, Spain, Modena, Bavaria, Saxony, Sweden, and Poland-Lithuania: We don't
Isn't there somebody you forgot to ask?
Much more than the War of the Spanish Succession, the 1740-1744 War of the German Unification would radically change the map of Europe, being considered the final nail in the coffin of whatever remained of feudal fragmentation, and starting a new era when all of Europe would be covered by sizeable, reasonably united entities. It also marked the definitive end of France as the uncontested main continental European power. Some historians even see in it the start of modern nationalism, given how, along the war, the Imperial side started to use what could be called proto-(German)nationalist rhetoric, downplaying religious differences in favor of a "national" cause to advance it's position.
Despite many not putting much faith on it, due to the religious differences, eventually, to the surprise of many (and disgust in Paris), the planned married of Friedrich II of Brandenburg-Prussia and Maria Theresia of Austria went ahead. At first, every relevant European power pretended to be fine with it, but, as soon as HREmperor Karl VI died, firce debates started on who should be the next emperor, with them eventually degenerating on debates questioning the legitimacy of Maria Theresia to inherit the Habsburg possessions, and the diplomatic acceptance of her marriage with Friedrich. Finally, in December 1740, a combined attack by the seven powers opposing the Imperial couple on their possessions, started the war that would completely change Western history.
The Imperial side would get support from the British, always eager to counterbalance France on the continent; the Dutch, distrutful of the French; the Russians, just out to grab some lands of the bordering belligerent states; and the Savoyards, keeping it's policy to oppose Spanish presence on northern Italy, and wanting to get in the good graces of the Imperial side.
The war of the German Unification ended in July 1744, with the Treaty of Aachen. It's term were:
-The unity of the Holy Roman Empire of the German Nation is reaffirmed, under Kaiser Friedrich IV and Kaiserin Maria Theresia, whose marriage is to be accepted by all states. It will be, from now, a hereditary monarchy under the house of Habsburg-Hohenzollern. It's vassals are now forbidden to engage in foreign policy on their own.
-The House of Hannover relinquishes it's German lands to the Imperial family (made both to strenghten it's hand, and for some concessions outside the German Empire).
-Saxony, Bavaria, and Osnabrück go to the Imperial family.
-The Wettins get to keep Poland-Lithuania, now to be a hereditary monarchy (the Russians, main winners of PLC's disfunction, oppose this, but this is supported by the Germans and the British, the first prefering to have a modestly strong mid-sized neighbour than to have Russian influence on their doors, and the second eager to keep the European balance of power).
-The Bavarian Wittelsbach get the Habsburg Netherlands and the Prince-Bishopric of Liege, that now becomes the Kingdom of Belgium, outside of the German Empire. Bouillon is to be it's vassal. If the Palatine branch of the family gets extinct, their lands won't go to the Belgian branch, but will return to the Imperial family. If the Belgian branch gets extinct, the Palatine branch will get Belgium, but their lands on the German Empire will go to the Imperial family.
-Lorraine, now elevated to a kingdom outside the German Empire, is returned to François Étienne and the house of Lorraine overall. Bar becomes a full part of this new kingdom.
-As a reard to their support, the Netherlands gets East Frisia. For the sake of map simplification, their disjointed lands in Limburg go to Belgium (less Catholics, also...)
-France loses Alsace to the German Empire, it going to the Imperial family, and the Three Bishoprics, annexed by Lorraine.
-The German Empire relinquishes it's claims to Italy.
-The Duchy of Savoy is elevated to a kingdom, the Kingdom of Lombardy, getting Parma, Milan, and Mantua from the Habsburgs in exchange for their support (also British pressure to promote a balance of power), and annexing Reggio and Mirandola from Modena. Monaco, Massa, and Modena are to be it's vassals.
-Tuscany is elevated to a kingdom, the Kingdom of Etruria, and given to Guiseppe, Prince of Ottajano (cadet branch of the Medicis) and his family. the Stato dei Presidi is annexed by it, and Piombino is to be it's vassal.
-Poland-Lithuania loses it's littoral provinces to the Kingdom of Prussia, that becomes a full part of the German Empire (one of the two kingdoms inside it, the other being Bohemia).
-The Spis mines are returned to Hungary.
-Poland-Lithuania loses Latgalia and West-Bank Ukraine to Russia. Courland becomes a Russian vassal.
-Sweden loses it's parto of Pomerania to the Imperial family, and a chunk of Finland to Russia. With Russian support, Adolf Friedrich of Holstein is elected king of Sweden.
-Germany and Hungary-Croatia are to be inherited by different male heirs of the Habsburg-Hohenzollern couple. If one of their coming lineages goes extinct, a union is alloed, but it must be undone as soon as the remaining lineage gets two male heirs. This point doesn't forbid an alliances between the two states (as the existing one between Bourbon France and Spain).
Along their four decades long reign, Friedrich IV the Great and Maria Theresia would proceed to strenghten Imperial institutions, and so, while at the end of their reign there was still a sizeable degree of decentralization, with it's many vassals, for the first time since the Middle Ages there was, in the heart of Europe, a generally coherent German state with an overall effective central government, clearly emerging as one of the main powers of the West.