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Wikipedia said:
While Emperor Ferdinand was occupied with the Bohemian rebellion of 1618, Bethlen led his armies into Royal Hungary in August 1619 and occupied the town of Kassa (Košice) in September, where his Protestant supporters declared him "head" of Hungary and protector of the Protestants. He soon won over the entirety of present-day Slovakia, even securing the capital of Royal Hungary, Pressburg (Bratislava), in October, where the palatine even handed over the Crown of St Stephen to Bethlen. Bethlen's troops joined with the troops of the Czech and Moravian estates (led by Count Jindrich Matyas Thurn), but they failed to conquer Vienna in November – Bethlen was forced to leave Austria after being attacked by George Druget and Polish mercenaries (lisowczycy) in Upper Hungary. Although he had conquered most of Royal Hungary, Bethlen was not averse to a peace, nor to a preliminary suspension of hostilities, and negotiations were opened at the conquered towns of Pressburg (Bratislava), Kassa (Košice) and Besztercebánya (Banská Bystrica). Initially, they led to nothing because Bethlen insisted on including the Czechs in the peace, but finally a truce was concluded in January 1620 under which Bethlen received 13 counties in the east of Royal Hungary. On 20 August 1620 the estates elected him King of Hungary at the Diet in Neosolium with the consent of the Ottomans, but Bethlen refused to accept the crown because he wanted to reconcile with the Habsburgs and reunite Hungary. However, the war with the Habsburgs resumed in Royal Hungary and Lower Austria in September.

What if Gabriel Bethlen and his allies succed in taking Vienna could it force the Habsburg to negoatiate a treaty and cut the 30YW short? If we see a peace treaty, the Habsburg would likely keep Croatia, while Gabriel Bethlen became king of Hungary, Frederick V would become king of Bohemia, but to not have two electorial votes, Pfalz would lose it status as Electorate, Austria or another of the Habsburg possesions could receive it instead securing the imperial throne for the Habsburgs. Of course the Habsburg would likely try their best to reghain their lost possesions, I could see the war restart with Gabriels death in 1629.
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