Offensive vs Defensive Realism
Both Offensive and Defensive Realism are offshoots of Realism, and to a more narrow extent, Structural Realism, also known as Neorealism. The two competing theories are derived from the same base arguments; an anarchial international system, no power is greater than the state, and a state’s greatest concern is its own security. The differences lie in how best to ensure security. Offensive realists believe the best way to ensure security (and sovereignty) is by gaining power at the expense of rivals, even if this requires the use of military action. Defensive realists believe the best way to ensure survival is by maintaining the current balance of power, and as conquest is typically more difficult than defense, states will choose to be defensive.
Offensive Realism
An offshoot of Structural Realism popularized by John Mearsheimer, it’s the belief that the international system creates powerful incentives for states to look for opportunities to gain power at the expense of rivals, and to take advantage of those situations when the benefits outweigh the costs. A state’s ultimate goal is to be the hegemon in the system. It is an aggressive system, where powerful states have an incentive to take advantage of weaker states before they become a peer rival. On the other hand, emerging powers will constantly seek to challenge the balance of power whenever the benefits outweigh the costs.
Defensive Realism
An offshoot of Structural Realism developed by Kenneth Waltz, the main goal is still the survival of the state. The key difference is that states under Defensive Realism have a greater incentive to maintain the status quo rather than challenging the balance of power. The view is seemingly anti-war, and discourages armed conflict except under defensive or exceptional means. It is also important to note the time periods in which each theory was popularized. Waltz’s theory of Defensive Realism was established during the Cold War, when there was a bipolar world order and near parity between US and Soviet military capabilities. Mearsheimer’s theory came during the late 1990s to early 2000s, when the United States was still enjoying the benefits of a unipolar system, but becoming increasingly wary of China’s emergence. In both cases, each theory accurately explained the ongoing era of geopolitics for their respective times. As Waltz would have predicted, the world never experienced direct confrontation between the militaries of the US and USSR, as it would cause too much disruption to each nation’s security, and in turn, survival. And now as Mearsheimer has predicted, China has relentlessly increased its own power in preparation for what many see as an inevitable clash with the United States over Taiwan. Meanwhile, the winner, if losses are not absolute, is set to gain undisputed hegemony in the international system.
References:
Mearsheimer, J. J. (2021). The Inevitable Rivalry: America, China, and the Tragedy of Great-Power Politics. Foreign Affairs , 100 (6), 48–58. https://www.jstor.org/stable/27121441
Mearsheimer, J. J. (2001) The Tragedy of Great Power Politics. W.W. Norton & Company
Waltz, K. N. (1988). The Origins of War in Neorealist Theory. The Journal of Interdisciplinary History , 18 (4), 615–628. https://doi.org/10.2307/204817