114 CHAPTER fouR ■ The InTernaTIonal SySTemUnited States in the immediate post– World War II era led to the greatest stability.^5
Other proponents of this theory, such as Robert O. Keohane, contend that hegemonic
states are willing to pay the price of enforcing norms, unilaterally if necessary, to ensure
the continuation of the system that benefits them. When the hegemon loses material
capability or is no longer willing to exercise its advantage in relative power, then sys-
tem stability is jeopardized.^6
It is clear, then, that realists do not entirely agree among themselves about the rela-
tionship between polarity and stability. Individual and group efforts to test this rela-
tionship have been inconclusive. The Correlates of War proj ect (discussed in Chapter 1)
did test two hypotheses flowing from the polarity- stability debate. J. David Singer and
Melvin Small hypothesized that the greater the number of alliance commitments in
the system, the more war the system will experience. They also hypothesized that the
closer the system is to bi polar ity, the more war it will experience. According to data
between 1815 and 1945, however, neither argument was proven valid across the whole
time span. During the nineteenth century, alliance commitments prevented war,
whereas in the twentieth century, proliferating alliances seemed to cause war.^7 Other
evidence from the 1970s suggests that although U.S. economic prowess declined in
relative terms, the international system itself remained stable; system stability is not
dependent solely on one power.^8realists and International system change
For realists, the nature of the change in the system can be reduced to the distribution of
peace and war between great powers (small and medium powers matter less). If that
structure affects the likelihood of war and peace in the system, then logically, any under-
standing of what causes structural change (e.g., in polarity) will result in an understand-
ing of what makes war or peace more likely. Changes in either the number of major actors
or the relative power of those actors may cause a fundamental change in the structure of
the international system. According to realists, wars are most often responsible for such
fundamental changes in power relationships. For example, World War II caused a rela-
tive decline of Great Britain and France, even though they were the victors. The war also
signaled the end not only of Germany’s and Japan’s imperial aspirations but of their
considerable military and economic capabilities as well. Their militaries were soundly
defeated; their civil societies were destroyed and their infrastructures demolished. Two
other powers emerged in dominant positions— the United States, now willing to assume
the international role it had shunned after World War I, and the Soviet Union, buoyed
by its victory, although eco nom ically weakened. The international system had funda-
mentally changed; the multipolar world had been replaced by a bipolar one.
Robert Gilpin, in War and Change in World Politics, sees another mechanism of
system change: states grow at uneven rates because states respond differently to po liti cal,ESSIR7_CH04_106_131_11P.indd 114 6/14/16 10:04 AM