that they are functionally undifferentiated by which he means that they are ‘alike in
the tasks that they face, though not in their abilities to perform them’.^6
If Waltz’s position is accepted, then his parsimonious definition of units creates the
potential for him to apply his theory across the course of world history. Moreover,
the application of his theory is left apparently untouched by the various attempts to
establish typologies based on the assumption that the nature of the state has undergone
very substantial transformations over time.^7 Unsurprisingly, however, the assumption
that it is possible to bracket the internal structure of the units has been challenged on
various counts, although all presuppose that the theory implicitly makes more assump-
tions about the nature of the units than Waltz acknowledges. Explicitly, Waltz argues
that the units are defined by a hierarchical political structure. Nexon argues, however,
that implicitly the theory makes additional and significant assumptions about the units:
that the ties between units are relatively sparse, that collective identification between
units is weak, that ties within states are dense and that collective identification within
states is strong. In other words, the theory rests on a kind of billiard ball model of
units.^8 But can this view of units be applied to the units that prevail in many historical
periods? Nexon argues that the dynastic states that operated in early modern Europe
simply don’t fit Waltz’s model because they had internal structures much looser and
more fragmented than those assumed by Waltz. Drawing both on the terminology
of historians who have examined this period, and on network analysis, he identifies
these units as composite statesand opens the possibility for comparison with units in
other historical periods that can also be embraced by this ideal type.
Osiander also critiques Waltz’s assumptions about units, arguing that his formu-
lation presupposes ‘homogeneous actors’ so that the resulting systems are made up
exclusively of, for example, tribes, empires or some other kind of unit. It follows that
although Waltz makes reference to a wide range of possible units that can interact in
an anarchy, his theory effectively precludes the possibility of heterogeneous systems
whereby firms, for instance, interact with street gangs or nations with tribes. But
Osiander insists that ‘heterogeneous not homogeneous systems have in fact been the
norm in western civilization’.^9 He is certainly not the first theorist to make this claim.
In the context of Europe, Tilly recognizes that empires, city-states and national states
co-existed for a period before the national states, because of their success in waging
war, eventually emerged as the dominant unit.^10 By contrast, Spruyt argues that three
different political forms emerged from feudalism: city-states and city leagues as well
as sovereign states. Sovereign states persisted, according to Spruyt, because they were
more successful at reducing transportation and information costs as well as being able
to produce the most credible commitments in the international system.^11
When one moves on to a world historical canvas, it becomes necessary to
accommodate a very much wider array of units than Waltz allows, and to accept
that these units have co-existed and interacted in many international systems. In a
rather crude attempt to provide a typology of units, which highlights the very
restricted nature of units identified by Waltz, we structurally differentiated units on
two dimensions, as mobile or immobile on one dimension and hierarchical and non-
hierarchical on the other (see Figure 17.1).
290 The paradox of parsimony