The Germiyanoğulları Principality was founded by Yakub Bey in one thousand two hundred and seventy-eight, centered in Kütahya in western Anatolia, during the time of the Anatolian principalities, which started with the collapse of the Anatolian Seljuk state. The era of Yakub Bey is considered the brightest period of the Germiyanoğulları principality. Yakub Bey, who started to act independently after 1300, was the closest rival of the Ottomans in the region. Probably, if the Ottoman Empire had not existed, the Germiyanoğulları Principality would have seen brighter times. All the lords in Aydın, İzmir, Isparta and Denizli declared their loyalty to Yakub Bey. However, since Yakub Bey wanted to subjugate all the Anatolian principalities, he went to Emir Çoban, the man of the Ilkhanid ruler Olcaytu Khan, and declared his loyalty to him. Then, the emir of Karahisar strengthened his position by becoming the son-in-law of Yakub Bey and began to serve as the governor of Karahisar for the Germiyans. From now on, Yakub Bey also had a falling out with his neighbors to the north, the Ottomans. According to the rumor in Ottoman sources, Rye Tatars, who were encouraged by Yâkub Bey, took advantage of Orhan Gazi's presence in Eskişehir and attacked the Ottoman lands and plundered the city and market of Karacahisar. The Germiyans were defeated in the war that broke out during these events. Yakub Bey's closeness to the Mongols and some of his betrayals disturbed the Ottoman side very much. It is estimated that Yakub Bey died in the 1340s. El-Ömerî records that the Germiyan Principality had a trained regular army during the reign of Yakub Bey.