Yoffee's essay "Collapse in Ancient Mesopotamia: What Happened, What Didn't," appears in a collection of essays called Questioning Collapse, in which different historians argue against Jared Diamond's thesis in his best-selling Collapse. Diamond contended that disregarding ecological constraints leads societies to collapse and disappear. Questioning Collapse, however, is premised on the idea that societies very rarely experience utter collapse per se. Instead, they morph and change, decline, adapt to circumstances, regenerate in various ways, and struggle along. In his particular essay, Yoffee contends that it was not environmental overreach that caused the decline of Assyria circa 700 BC but excessive political and military expansion. This led to authority being spread too thin with the development of provincial ties to the central Assyrian culture that were weak and tenuous. Thus, when the empire started to experience defeat, it lacked both the material and cultural capital to hold itself together. It had imported many workers from other cultures, and when material conditions began to deteriorate, people within Assyria itself began to rebel. As the situation unraveled, it was lack of cultural capital—lack of enough people who were truly culturally Assyrian—that eventually defeated the Assyrians to the point that they could not regenerate or rise again.

I assume that you are asking about Norman Yoffee’s chapter in the book Questioning Collapse, which he edited along with Patricia McAnanay.  In this chapter, Yoffee presents an argument about the collapse of the Assyrian state that, unlike the argument presented by Jared Diamond in Collapse, has nothing to do with environmental degradation.

According to Yoffee, the main reason for the fall of the Assyrian state and its inability to regenerate was what might be called overreach.  In other words, it was the imperial policies of the Assyrian kings that caused their state to collapse.  Yoffee says that the mistake that the Assyrian kings made was to bring in too many non-Assyrians from conquered lands.  He says this meant that much of “Assyrian” society was made up of people who had no connection to Assyria or its rulers.  What this meant was that, when the Assyrian rulers were defeated, there was no one left to regenerate Assyrian society.  In the past, Assyrian society had really been Assyrian.  Therefore, when rulers and elites were defeated, a new set of Assyrians would rise to the top and become a new elite that was still Assyrian.  But now, the bottom layers of society were not really Assyrian anymore.  When the top layers were defeated, no new elite was created and the Assyrian state did not regenerate.