Around 1000 BC, after a decline at the end of the Middle Assyrian Period, rulers of Ashur began to lead military expeditions west and north. These were campaigns for tribute, booty and resources such as metal and horses.
Under Ashurnasirpal II (883-859 BC) the capital was moved to Kalhu (Nimrud) and the rulers that followed expanded the kingdom of Assyria to the River Euphrates and beyond. With the accession of Tiglath-Pileser III in 745 BC, changes in the administration of Assyria included the take over of defeated countries to form an empire. Over the next 100 years kings such as Sargon, Sennacherib and Esarhaddon not only built new capitals (Khorsabad and Nineveh) but expanded the empire until Assyrian control stretched from Iran to the Mediterranean.
Under Ashurbanipal (668-627 BC) problems on the borders in Egypt, Babylonia and Elam were crushed. However, by 616 BC a newly established king of Babylon, Nabopolassar, was in a position to launch attacks on Assyria. It was only when he made an alliance with the Medes from Iran that the great cities of Assyria fell. In 612 BC Nineveh was sacked and the Assyrian court fled west to the city of Harran where in 609 BC they were defeated by the Babylonians who took over much of the empire.