Postclassical realism is an international relations theory term coined by academic Stephen Brooks. It refers to a strand of realist international relations theory scholarship distinguishable to the neorealist scholarship of Kenneth Waltz. It is also roughly equivalent to defensive realism.
Contrary to neorealism, it asserts that states do not necessarily always maximize their security. Rather than focusing on the possibility of conflict, they place more emphasis on the probability of conflict. Under this premise, conflict is more likely to occur when circumstances favor offensive capabilities in military technology and geography over defensive capabilities. When the opposite is true, states will discount the short-term gains of a military buildup in favor of the long-term benefits from economic growth. In this sense, states are ultimately power seekers who only maximize security when threats are very high. Unlike classical realism, however, postclassical realism doesn't assume that power is an end in and of itself; rather, power enables states to have the most flexibility to accomplish any goals they deem important.