He argued that looking at the inner working of nation states, or as he stated the "behavior of units," was counter-productive to assessing a system of nation states as a whole (Keohane, 1986). Instead, Waltz argued that one must look at the greater system in play rather than the individual "units" so as to be able to make realistic and correction conjectures and theories about world politics (Keohane, 1986). Difference between Structure & Units

In a neorealism context, the difference between structure and units is fairly clear. The basic structure of a structure is usual anarchic in nature whereas the individual nation states (the units) are biased towards survival and their ongoing success (Keohane, 1986). Because there...

Any illusion of civility and structure in a single nation state does not extend to the international sphere…not in the slightest. This is the basic view of neorealists like Keohane.
References

Keohane, R.O. (1986). Neorealism and its critics. New York: Columbia University Press.