Count Philip III had reunited the Nassau lands after several centuries of division. His two sons, Sigismund and Philip would have probably been expected to divide the county once more between them. However the Polish Sejm, in one of its fits of unexpected behaviour, elected Sigismund to the Polish throne in 1508 thanks to his wife. Finding his hands more than full with the issues of Poland-Lithuania, Sigismund let Philip take the entire county of Nassau when their father died in 1513. His single surviving daughter, Amalia, had married Wenceslaus II of Luxembourg-Hungary-Bohemia.Both brothers soon found themselves without a male successor. Sigismund had proved himself to be an able and respected king and his daughter Anna was elected 'King' (there was no provision to elect queens in Poland) on his death in 1543. While that may have been fine for Poland the German states took a more concerned view especially after Philip IV died after falling down the stairs in Nassau castle in 1544. He had left no surviving children. Nassau had become a hotbed of Calvinism and Anabaptism which Philip IV had done little to stem and even occasionally shown signs of sympathising with. Other Nassau claimants abounded but seeing as most were now rulers of Lutheran states the Catholic Empire sided with Luxembourg on the issue. Emperor Albert had no desire to see another state join the breakaway northern empire. Equally Albert had no desire to see Nassau governed by Poland seeing as it had recently embraced religious tolerance. God forbid this should be extended into Germany. Salian law, which would rule out both Luxembourg and Poland's claims was abandoned. Luxembourg was in a more parlous state than many realised. Wenceslaus II had not received the Imperial title after the death of his father and this had ruined much of his credit-worthiness. Furthermore his death in 1540 had left his long-suffering daughter Joanna acting as regent for her step-brother; Amalia's only child, Henry of Prague. While Bohemia and Hungary stayed more of less loyal its holdings in Brandenburg and the Low Countries were riven by religious issues. It was also still repaying the debts from the War of Anglian Succession and fighting Venice along the Dalmatian Coast. Many thought it could ill-afford another conflict. However the desire to hold back the tide of Protestantism was enough to secure Henry of Prague as the rightful heir to the throne.
Poland however, free from foreign entanglements, apart from its intermiable wars against Tver, was more than willing to back up its support for Anna I and her young son Prince Sigismund.
Luxembourg's Army of Flanders marched into Nassau in September 1544. The nobles there had been expecting it but were still split over who exactly they should be welcoming as their new lords. Most of the Catholic nobles soon rallied to Luxembourg's side and by December only a few Protestant nobles held out, possibly hoping for one of their Protestant neighbours to intervene on their behalf. None did, but this 'failure' would be a big driver in forging the Schmalkaldic League the following year. In the meantime Joanna made little hesitation in sending in her clerical big guns to eradicate Lutheranism amongst the populace. Not withstanding continued religious troubles the war in Nassau itself was over by the Spring of 1545. In the east it was merely just beginning however.
Blocked from securing Sigismund's birth-right directly, the Polish nobles set their sights on other targets. As Luxembourg mopped up in Germany the Poles had invaded Brandenburg and Bohemia whilst the Lithuanian nobles took on Hungary. The Hungarian magnates, already much harried by tax and failures in Dalmatia failed singularly to organise an effective defense and Kosice was lost after a short siege, giving the Lithuanians access to the Hungarian plain. The war in Bohemia was mainly characterised by the two armies trying to force the other into a decisive battle on their terms. Ostrava was put under siege although it would not fall. Neither would the Bohemian army be cornered. In Brandenburg the Poles had a more testing time. For the whole of 1546 Poland effectively dictated the war.
In 1547 outside forces intervened to end the war. Tver once again started activities on the borders. For the Lithuanian nobles their own estates were much more important than some small distant German territory. Still undefeated in Hungary they withdrew to begin a long march eastwards. The Polish army now rushed to secure Kosice but were overstretched in Bohemia. The Luxembourg's belated victory came at Opava on 15th May. With the Sejm's enthusiasm for the war rapidly fading away Anna sued for peace. Anna (and Sigismund) agreed to renounce their claims to Nassau. In return Bohemian troops were promised for Poland's use against Gothenland and Kalmar if they should need it.
Hungary seethed with anger over the seeming pro-Bohemian stance of the Luxembourg army during the war. Bohemia's nobles meanwhile saw the increased oppression the Lutherans were receiving in the Low Countries. Both kingdoms were . It did not take much to push them into open revolt. When Henry of Prague died in 1550 Joanna assumed her own son, Henry of Ghent, would be confirmed as the rightful king. They did not however, almost tendering out the title to other great lords and eventually settling on the dynamic Archduke of Austria, Maximilian. Almost bankrupt Luxembourg was powerless to oppose it. Hungary stayed loyal until Henry of Ghent assumed the throne as Henry VIII, and then promptly embraced Lutheranism. The Hungarian nobles were appalled. In response they threw the remaining Luxembourg officials out of Buda and elected one of their own, Stephen Zápolya as king.
Poland meanwhile seemed to bounce-back from the war. The conflict against Tver was quickly over, Polish victories at Kolomna and Rzew saw to that. And in 1551 Anna I oversaw the creation of a true union between Poland and Lithuania; the Commonwealth, which elevated the Lithuanian nobles to the same level as their Polish peers. This new arrangement kept the country internally peaceful, at least until military defeat in the Fifty Years War demanded further political reform.
The county which started the conflict, Nassau, was quietly incorporated into the Luxembourg realm. Luxembourg slowly rebuilt its treasury with a new focus on trade, especially with Tawantinland and, by staying studiously neutral in the new Germany, split between the Catholic Empire and its upstart Protestant rival, the Schmalkaldic Empire. Luxembourg's preoccupation with its own war had deprived Emperor Albert of its potential resources when facing the Lutheran armies of the Schmalkaldic League and had been forced to concede authority in Northern Germany after several defeats. Germany would remain divided along confessional lines for another century.