The Kassite conquerors of Babylonia soon submitted to the influences of Babylonian civilisation.  Like the Hyksos in Egypt, they adopted the manners and customs, the writing and language, of the conquered people, sometimes even their names.  The army, however, continued to be mainly composed of Kassite troops, and the native Babylonians began to forget the art of fighting.  The old claims to sovereignty in the west, however, were never resigned; but the Kassite kings had to content themselves with intriguing against the Egyptian government in Palestine, either with disaffected Canaanites, or with the Hittites and Mitannians, while at the same time they professed to be the firm friends of the Egyptian Pharaoh.  Burna-buryas in B.C. 1400 writes affectionately to his “brother” of Egypt, begging for some of the gold which in Egypt he declares is as abundant “as the dust,” and which he needs for his buildings at home.  He tells the Egyptian king how his father Kuri-galzu had refused to listen to the Canaanites when they had offered to betray their country to him, and he calls Khu-n-Aten to account for treating the Assyrians as an independent nation and not as the vassals of Babylonia.

The Assyrians, however, did not take the same view as the Babylonian king.  They had been steadily growing in power, and had intermarried into the royal family of Babylonia.  Assur-yuballidh, one of whose letters to the Pharaoh has been found at Tel el-Amarna, had married his daughter to the uncle and predecessor of Burna-buryas, and his grandson became king of Babylon.  A revolt on the part of the Kassite troops gave the Assyrians an excuse for interfering in the affairs of Babylonia, and from this time forward their eyes were turned covetously towards the kingdom of the south.

As Assyria grew stronger, Babylonia became weaker.  Calah, now Nimrud, was founded about B.C. 1300 by Shalmaneser I., and his son and successor Tiglath-Ninip threw off all disguise and marched boldly into Babylonia in the fifth year of his reign.  Babylon was taken, the treasures of its temple sent to Assur, and Assyrian governors set over the country, while a special seal was made for the use of the conqueror.  For seven years the Assyrian domination lasted.  Then Tiglath-Ninip was driven back to Assyria, where he was imprisoned and murdered by his son, and the old line of Kassite princes was restored in the person of Rimmon-sum-uzur.  But it continued only four reigns longer.  A new dynasty from the town of Isin seized the throne, and ruled for 132 years and six months.

It was while this dynasty was reigning that a fresh line of energetic monarchs mounted the Assyrian throne.  Rimmon-nirari I., the father of Shalmaneser I. (B.C. 1330-1300) had already extended the frontiers of Assyria to the Khabur in the west and the Kurdish mountains in the north, and his son settled an Assyrian colony at the head-waters of the Tigris, which served to garrison the country.  But after the successful revolt of the Babylonians against Tiglath-Ninip the Assyrian power decayed.  More than a century later Assur-ris-isi entered again on a career of conquest and reduced the Kurds to obedience.