Ibbi-Sin (=Ibbi-Suen) in Wikipedia
Ibbi-Sin, son of Shu-Sin, was king of Sumer and Akkad and last king of the Ur III dynasty, and reigned circa 1963 BC-1940 BC (Short chronology). During his reign, the Sumerian empire was attacked repeatedly by Amorites. As faith in Ibbi-Sin's leadership failed, Elam declared its independence and began to raid as well.
Ibbi-Sin ordered fortifications built at the important cities of Ur and Nippur, but these efforts were not enough to stop the raids or keep the empire unified. Cities throughout Ibbi-Sin's empire fell away from a king who could not protect them. Ibbi-Sin was, by the end of his kingship, left with only the city of Ur. In 1940 BC, the Elamites, along with "tribesmen from the region of Shimashki in the Zagros Mountains" (Stiebing 79) sacked Ur and took Ibbi-Sin captive; he was taken to the city of Elam where he was imprisoned and, at an unknown date, died.
The success of the Amorite invasion
The Amorites were considered a backwards people by Mesopotamian standards. That they were able to cause so much trouble in the Ur III empire is surprising. In truth, the Amorite efforts to invade the empire may have been effective simply because they were in the right place at the right time. Scholars have suggested that, by the reign of Ibbi-Sin, the empire was already in decline due to long-term drought--in fact, the same drought that helped to take down the Akkadian Empire circa 2193 BC may have been responsible for the fall of Ur III. On page 79, Stiebing writes of evidence supporting this assertion:
"Studies of Persian Gulf sediments indicate that the stream flow of the Tigris and Euphrates was very low around 2100-2000 B.C.E. [...] Any damage to the agricultural system by enemy raids, bureaucratic mismanagement, or an inattentive ruler would result in food shortages"
In years seven and eight of Ibbi-Sin's kingship, the price of grain increased to 60 times the norm. From this, we can conclude that the success of the Amorites in disrupting the Ur III empire is, at least in part, a product of attacks on the agricultural and irrigation systems; these attacks brought famine and caused an economic collapse in the empire, paving the way for the Elamites to strike into Ur and capture the king.