In a diploma, dated July 26, 1324, King Charles I of Hungary refers to Basarab as "our voivode of Wallachia" which indicates that at that time Basarab was a vassal of the king of Hungary.^[62] In short time, however, Basarab refused to accept the suzerainty of the king, for neither Basarab's growing power nor the active foreign policy he was conducting on his own account to the south could be acceptable in Hungary.^[63] In a new diploma, dated June 18, 1325, King Charles I mentions him as "Basarab of Wallachia, unfaithful to the king's Holy Crown" (Bazarab Transalpinum regie corone infidelem).^[64]

Hoping to punish Basarab, King Charles I mounted a military campaign against him in 1330. The king advanced with his host into Wallachia where everything seemed to have been laid waste. Unable to subdue Basarab, the king ordered the retreat through the mountains. But in a long and narrow valley, the Hungarian army was attacked by the Romanians, who had taken up positions on the heights. The battle, called the Battle of Posada, lasted for four days (November 9–12, 1330) and was a disaster for the Hungarians whose defeat was devastating.^[65] The king was only able to escape with his life by exchanging his royal coat of arms with one of his retainers.^[66]

The Battle of Posada was a turning point in Hungarian-Wallachian relations: though in the course of the 14th century, the kings of Hungary still tried to regulate the voivodes of Wallachia more than one time, but they could only succeed temporarily. Thus Basarab's victory irretrievably opened the way to independence for the Principality of Wallachia.