Originally, Nassau was a county in Germany near Hesse. Its history started to diverge from OTL when in 1254 Count Otto I was killed after getting in trouble with the Teutonic Order. His brother Walram II got all of Nassau - in TTL, it was never divided.
During the following centuries, thanks to some good marriages arranged by the counts Adolf II (1357-66) and Kraft I (1419-25), the county almost doubled in size, united with the counties of Waldeck and Mark, plus some smaller areas.
During the Twenty-Year War, Nassau under count Kraft II acquired (thanks to some smart alliances) a part of Mainz in Hesse, a part of the electorate of Trier around Koblenz, Dortmund, and the diocese-duchy of Westphalia (formerly owned by the archbishop of Cologne). From now on, they controlled the electorate of Mainz.
In 1581/82, the Gottesfreistaat Münster attacked the little country of Oldenburg, conquered it. The Netherlands and Nassau felt seriously threatened, asked the Triple Monarchy and France for support. In the Netherlands this doesn't work out because their king died in an unfortunate moment, but Nassau received help (French musketeers) and defeated Münster, annexed the territories of Tecklenburg, Ravensberg, Paderborn. The religious dissenters who didn't want to reconvert fled, some of them even to Atlantis.
When François IV was in power, Nassau would ally with him and fight in two wars, the Luxemburgian War of Succession and the anti-French War. With French help, Nassau defeated the Gottesfreistaat Münster during 1676-79, annexed it, which made Nassau one of the biggest states in Germany.
1722-34, Ignaz IV of Nassau was Holy Roman Emperor.
Grand duke Adolf Ignaz II (nicknamed "Adolf Nazi") died in 1830. While Britain had some claims for for the throne, a plebiscite about the future in the country made Nassau a republic, which was soon united with Germany (which thus gained the important Ruhr area). King Philip of Britain who nominally was Holy Roman Emperor, wasn't too happy about this development.
|List of Roman kings of the HRE|
|Nations that joined the HRE later: Poland | Prussia|
|Nations that left the HRE: Bohemia | Florence | Switzerland|
|Nations that became defunct|