Battle of Nihriya
The Battle of Niḫriya marked the peak of hostilities between the Hittites and the Assyrians over control of the remnants of the Mitanni empire. After Hittite king Šuppiluliuma I conquered Mitanni, he created the provinces of Aleppo and Carchemish, distributing much of Mitanni's former territories among his allies. The remaining part of Mitanni retained its independence as a Hittite vassal state called Ḫanigalbat. However, during the reign of Hittite king Mursili III, Ḫanigalbat was conquered by the Assyrian Empire, which controlled the east bank of the Euphrates. When Ḫattusili III deposed his nephew Urḫi-Tešub and took the Hittite throne, he had to accept the permanent loss of Ḫanigalbat to the Assyrians.
Assyrian involvement in Syria continued under King Sulmanu-ašared I, leading to a crisis with the Hittite Empire. The Hittites saw Assyrian expansion as a direct threat and went to battle under King Tudḫalia IV, Ḫattusili's son and successor. This major conflict, known as the Battle of Niḫriya, is detailed in a letter (RS 34.265) sent by Sulmanu-ašared to Ugarit and is further supported by Hittite document KBo IV 14, indicating that the battle occurred around the 20th year of Sulmanu-ašared's reign. Contrary to earlier beliefs, Niḫriya was located in the Upper Balikh region, not along the Upper Tigris.
The Assyrians won a decisive victory at Niḫriya, which severely destabilized the Hittite state. King Tudhaliya IV faced several internal revolts but ultimately managed to retain his throne. Hostilities between Assyria and Ḫatti continued for about five years before a peace was negotiated, which brought a temporary end to their conflict.