A royal charter issued on 26 July 1324 is the first document to reference Basarab. According to the charter, he was subject to Charles I of Hungary as the voivode of Wallachia. Basarab became "disloyal to the Holy Crown of Hungary" in 1325. He seized the Banate of Severin and raided the southern regions of the Kingdom of Hungary. Basarab supported Michael Shishman of Bulgaria's attack against the Kingdom of Serbia, but their united armies were defeated in the Battle of Velbazhd on 28 July 1330. Soon after, Charles I of Hungary invaded Wallachia, but the Wallachians ambushed and almost annihilated the royal troops in the Battle of Posada, which occurred between 9 and 12 November 1330.