Their success was built upon the relative political stability that the Kassite monarchs achieved. They ruled Babylonia practically without interruption for over four hundred years- the longest rule by any dynasty in Babylonian history. Even after a minor revolt in 1333 BC and a seven-year Assyrian hiatus in the 13th century BC, the ruling Kassite family managed to regain the throne.
The transformation of southern Mesopotamia into a territorial state, rather than a network of allied or combatative temple-cities, made Babylonia an international power. Kassite kings established trade and diplomacy with Assyria, Egypt, Elam, and the Hittites, and the Kassite royal house intermarried with their royal families. There were foreign merchants in Babylon and other cities, and Babylonian merchants were active from Egypt (a major source of Nubian gold) to Assyria and Anatolia. Kassite weights and seals, the packet-identifying and measuring tools of commerce, have been found in Thebes in Greece, in southern Armenia, and even in a shipwreck off the southern coast of Turkey.
The Kassite kings maintained control of their realm through a network of provinces administered by governors. Almost equal with the royal cities of Babylon and Dur-Kurigalzu, the revived city of Nippur was the most important provincial center. Nippur, the formerly great city, which had been virtually abandoned about 1730 BC, was rebuilt in the Kassite period, with temples meticulously re-sited on their old foundations.
Other important centers during the Kassite period were Larsa, Sippar and Susa. Even after the Kassite dynasty was overthrown in 1155 BC, the system of provincial administration continued and the country remained united under the succeeding rule, the Second Dynasty of Isin. Comparisons of Kassite Babylonia with Feudalism are now considered more misleading than useful, but the prestige of Nippur was enough for a series of 13th century Kassite kings to reassume the title 'governor of Nippur' for themselves.
Documentation of the Kassite period depends heavily on the scattered and disarticulated tablets from Nippur, where thousands of tablets and fragments have been excavated. They include administrative and legal texts, letters, seal inscriptions, kudurrus (comparable to land grants and administrative prerogatives), private votive inscriptions, and even a literary text (usually identified as a fragment of a historical epic).
Kassite rulers in Babylon were also scrupulous to follow existing forms of expression, and the public and private patterns of behavior "and even went beyond that - as zealous neophytes do, or outsiders, who take up a superior civilization - by favoring an extremely conservative attitude, at least in palace circle." (Oppenheim 1964, p. 62). In the course of centuries, however, the Kassites were absorbed into the Babylonian population. Eight among the last kings of the Kassite dynasty have Akkadian names, and Kassite princesses married into the royal family of Assyria.
Under the Assyrian king, Ashur-Dan, the last Kassite king was driven from Babylonia in the twelfth century BC, and the "interregnum" in Mesopotamia came to a close.
The Kassite tribe of Khabira seems to have settled in the Babylonian plain. Remnants of Kassite tribes were living in the mountains northwest of Elam, immediately south of Holwan, when Sennacherib attacked them in 702 BC. They are doubtless the "Kossaeans" of Ptolemy, who divides Susiana between them and the Elymaeans. Alexander the Great battled Kossaeans in the winter of 323 on his way from Ecbatana to Babylon; according to Strabo (xi. 13,3,6) the Kossaeans were the neighbours of the Medes. Th. Nöldeke (Gott. G. G., 1874, pp. 173 seq.) has shown that they are the Kissians of the older Greek authors who are identified with the Susians by Aeschylus (Choephorae 424, Persians 17, 120) and by Herodotus (v. 49, 52).