Why nuclear superiority matters for compellence

Iranian foreign minister Mohammad Javad Zarif,  speaks with E.U. high representative Catherine Ashton as U.S. Secretary of State John Kerry embraces France’s Laurent Fabius in Geneva. (Fabrice Coffrini/AFP/Getty Images)
As diplomats work on a comprehensive nuclear deal to keep Iran from the bomb, Russia’s nuclear saber rattles in ways not seen since the 1980s, and North Korea, China, India and Pakistan expand and modernize their nuclear arsenals, the role of nuclear weapons is returning to the center of both foreign policy and scholarly debates.  Some of the most basic questions both groups would like to answer include: Do nuclear weapons matter? More specifically, can countries use nuclear weapons to deter and compel adversaries short of launching a nuclear attack? And, finally, does nuclear superiority, an advantage in the size and sophistication of a nuclear arsenal relative to that of an opponent, provide a coercive edge?
The answer to these questions will help inform our understanding of several vital questions: how easy it will be to persuade Tehran to forego a nuclear weapons capability; to what lengths the international community should go to put verifiable limits on Iran’s nuclear program; how concerned Washington should be about an increased emphasis on nuclear weapons in potential adversaries’ posture and doctrine; and whether the United States should remain committed to maintaining a robust nuclear arsenal.
To better inform these deliberations, this post reports the results of our new academic paper that finds that nuclear weapons are indeed an important factor in international politics. In particular, it shows that nuclear superiority increases a state’s ability to engage in international compellence.
(For those who may be wondering, compellence is typically defined as a military threat designed to change the international status quo and is generally thought to be more difficult than deterrence, a military threat to preserve the status quo).
The previous state-of-the-art in academic studies on this issue was reviewed in The Monkey Cage in a mini-symposium earlier this year. One study showed that nuclear-superior states are more likely to achieve their basic goals in international crises, while another demonstrated the opposite: Nuclear-armed states are no more likely to issue successful compellent threats than similar non-nuclear states. In other words, the results were inconclusive.
We decided to investigate this matter further. We saw several shortcomings with the study that found that nuclear weapons don’t matter, and our research sought to improve upon these limitations.
Using the exact same data (the Militarized Compellent Threat dataset) as the previous study, our basic finding is unambiguous: The nuclear balance of power is fundamental to patterns of international compellence.
To begin, we show that the previous study correctly reported that the rate of compellent success for non-nuclear challengers since 1945 (16 compellent successes in 69 attempts) is not statistically different from the success rate of nuclear-armed challengers (10 of 49). Unfortunately, however, the study also overlooked much of what is interesting about nuclear weapons’ effect on compellence.
First, nuclear weapons embolden states to attempt compellence in the first place. Nuclear-armed states attempt compellence roughly once in every 1,000 opportunities, whereas non-nuclear states have attempted compellence only about once in every 16,000 opportunities.
Second, nuclear-armed states are much more likely to achieve compellent success precisely because they are more willing to try. In social scientific terms, there is a large “selection effect.” The ratio of compellent successes to possible opportunities is roughly one in 5,000 for nuclear-armed states, but only one in 69,000 for non-nuclear states. As soccer coaches often quip, you miss all of the shots you don’t take.
Third, these effects are most evident when narrowing our focus to dyads (pairs of states) with at least one nuclear power. After all, these are the most relevant cases for assessing the effect of nuclear superiority on compellence. We find that nuclear-superior states (states that enjoy a nuclear advantage either because they possess nuclear weapons and their opponent does not, or because they possess more nuclear warheads than a nuclear-armed adversary) have attempted 49 compellent threats and successfully compelled opponents in 10 separate episodes.
In contrast, nuclear-inferior states (non-nuclear states facing nuclear-armed adversaries or nuclear-armed states against opponents with larger nuclear arsenals) have only even attempted compellence three times and were successful only once. Moreover, in our follow-up examination of this single success (Turkey supposedly compelling Great Britain as they were working together to restore order on Cyprus in 1963), we were unable to find any evidence of a militarized compellent threat, to say nothing of a successful one, leading us to doubt whether an inferior state has ever successfully compelled a nuclear-superior opponent.
Our finding does not result because certain aggressive countries build large nuclear arsenals and bully other states. Rather, we show that the exact same countries behave very differently depending on whether they are in a position of superiority or inferiority. The table below lists the compellent threats issued by nuclear-armed states from 1945 to 2001. The column on the left lists the compellent threats made by each state against countries with more nuclear weapons and, on the right, the compellent threats against states with fewer nuclear weapons.
 

The results are stunning. Since 1945, nuclear-armed states have attempted to compel inferior states 49 times and have never once tried to compel a superior state. In other words, for nuclear powers, nuclear superiority is a necessary condition for even attempting compellence.
In sum, nuclear-superior states are much more likely to achieve compellent success because they are more willing to try. In this way, nuclear compellence is not that different from other aspects of life. As Thomas Edison put it, “The most certain way to succeed is always to try just one more time.”
Matthew Kroenig and Hans Noel are associate professors in the Department of Government at Georgetown University. Miriam Krieger is a PhD student in the department.