He was the eldest surviving son of Muwatalli II. He was a grandson of Mursili II. The noted Hittologist Trevor Bryce credits this king with a reign of only 5 years and dates him at 1272 BC – 1267 BC. However, Mursili III almost certainly ruled the Hittite Empire for 7 years, as his successor Hattusili III explains in an inscription justifying the latter's seizure of power from this king. Mursili III must, hence, be dated from ca. 1272–1265 BC. The reigns of his successors should also be downdated by two years in Trevor Bryce's chronological table for the Hittite kings. During his reign, Mursili III reverted the capital from Tarhuntassa back to Hattusa. However, the Assyrians captured Hanigalbat, which severely weakened his legitimacy to rule over the Hittite Empire. In his seventh year, Mursili III attacked and seized control of his uncle Hattusili's regional strongholds of Hakpissa and Nerik within the Hittite Empire in order to remove Hattusili as a threat to the throne. Hakpissa served as the centre of Hattusili's power while Nerik was under Hattusilis's sway from the latter's position as high priest there. Hattusili then states in a well-known text: Consequently, Mursili III's reign was seven years. In the subsequent revolt, Hatusilli gathered a considerable force. This included natural allies from his local strongholds of Nerik and Hakpissa, as well as many non-aligned Hittites who were impressed with his record of service to the Hittite Empire. His strategic military victory over Ramesses II of Egypt in the 1274 BCBattle of Kadesh was favourably contrasted with the rather "undistinguished and largely unproven occupant of the throne of Hattusa" – Urhi-Teshub/Mursilis III – who had lost Hanigalbat to Assyria in his reign. Hattusili's forces even included elements of the Kaska peoples who were sworn enemies of the Hittites. Hatusilli quickly defeated Mursili III and seized the throne from his nephew; he then succeeded to power as King Hattusili III. After his victory, Hattusili appointed Mursili's brother or brother-in-law, Kurunta, as the vassal king over Tarhuntassa in order to win the latter's loyalty. Mursili fled to Egypt, the land of his country's enemy, after the failure of his plots to oust his uncle from the throne. Hattusili III responded to this event by demanding that Ramesses II extradite his nephew back to Hatti. This letter precipitated a crisis in relations between Egypt and Hatti when Ramesses denied any knowledge of Mursili's whereabouts in his country and the two empires came dangerously close to war. However, both kings eventually decided to resolve the issue by making peace in Year 21 of Ramesses II. An extradition clause was also included in the treaty. Mursili III soon thereafter disappears from history after his sojourn in Egypt. Mursili III seemingly had a son. A certain Hartapucalls himself a great king and son of the great king Mursili, who normally is identified with Mursili III. Hartapu likely has ruled Tarhuntassa in the 2nd half of the 13th century BC.
