I'm sorry but your maps are of low quality (let's take only the one that calls Bohemia "Austrian Dominion", judging from the style of it, It's maybe a product of the XIX century Habsburg manufacturies. On my map you can see the publisher on the top left. Next one is the photoshop version of that antique map, the other 3 arguably edited from a modern map with windows paint.)
But if you don't believe me search for Várad(Oradea in Romanian) false peace or Edirne peace (1444), please take your time!
Serbia ceased exist in 1439. Despot Brankovc fled to his Hungarian lands. The Serbian Despot was always a vassal of the Hungarian ruler since 1389.
In 1389 in the battle of Rigómező (in English Kosovo I think) Serbia were dissolved as a sovereign state, Despot died. Smaller lords of the southern lands became Ottoman vassals, while on the north Lazarevic became despot & enjoyed Hungarian protectorship. In 1403 He grabbed Nándorfehérvár/Belgrád for his assistance in a civil war to Sigismund.
In 1426 Sigismund asked Lazarevic to give back Nándorfehérvár for defensive purposes, the King gave him instead the dominions of Szatmár, Világos, Csepel, Németi and Szolnok, lands worth of 50k gold annual income. This happening looks immidiately different when we look at what happened, when Durad Brankovic stepped up to be Despot. Venice occupied the Dalmatian shores, Ottomans occupied Albania. He had only one option to go for, Sigismund.
The King was like waiting for that moment, and guaranteed him and Serbia. In 1428 a crusade began with mostly Hungarian soldiers. The army were beaten near Galambóc, even Zsigmond hardly escaped the scenery. Brankovic had to make peace with the Padisah, who demanded 50k gold annually and 3k cavalry ready on a call to arms plus he had to gave two of his sons to Murad II as hostages. So techincally the costs waged on Brankovic with this war were payed by Sigismund, while diplomatically defending northern Serbia from Ottoman influence.
So you can see, Sigismund gave Hungarian lands to Brankovic to secure his alliegence and Hungarian armies manned the forts along river Sava. Any other parts of Serbia were occupied by Ottomans or Venetians. In this circumstance you cannot say, that Serbia was a sovereign state both of 1438 (F.E.) or in 1428. The despot personally attended Hungarian Diets and accepted 3 separate Hungarian rulers as overlords.
He also got unique rights from the King also. (F.E. Serbian Orthodox temples were built in Ráckeve and Szentendre where some of Serbia's population fled).
In this bargain Brankovic became THE biggest noble in Hungary. Brankovic inherited every land he held and respected every oath he made.
Let's jump in time.
The year is 1437, Sigismund died, with claiming Albert von Habsburg as heir if he organizes a crusade. Next year Albert gathered a large army and marched south. Ottomans were faster organizing and had already laid siege on the fort of Szendrő, while Albert was camping in Titel (where river Duna and Tisza unites). Unfortunately disease broke out in the camp (the one that made people vomit and shit blood), the King was infected and died too the army were disbanded.
Brankovic was an open supporter of this crusade, so Ottoman avenge came. Both sons of Brankovic were blinded, and Serbia was pillaged. Brankovic had to fled to Szatmár, Hungary.
Before telling you why Serbia despite all I wrote was a sovereign state in 1444 nov 11, now do you agree that Brankovic was a vassal of Hungary in for example 1439?
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