After a long absence, revolutions are reclaiming an important place in the study of international relations. Of course, in the mid-1980s, many opined that the era of revolutions was over. With most of the world ruled by constitutional regimes, modernizing dictatorships, or party-states, all of which seemed secure from popular overthrow, the notion that revolutions of the kind that had overturned monarchies and empires in the past would recur on a significant scale seemed quaint.

Even when massive popular protests led to the fall of communist regimes in Eastern Europe and the Soviet Union, they were treated not as revolutions, but as something else—“refolutions” perhaps, or “nonviolent” regime changes. Yet when these were followed by another wave of popular protests overturning regimes in the Philippines, Georgia, Ukraine, and Serbia, it became clear that regimes once thought secure against mass uprisings were not. And when a wave of revolutions, some extremely violent, swept the Middle East and North Africa in 2010–11, it became impossible to ignore the fact that revolutions were still very much a dynamic part of world politics. True, more of these events were nonviolent urban revolutions, and only a few led to violent civil wars; but they were clearly events in which a combination of mass protests and elite defections brought down governments and produced a change in regime. In other words, they were surely revolutions, even though, as Mark Beissinger (The Revolutionary City, 2022) has persuasively demonstrated, the type and character of revolutions has “evolved.”

And these revolutions did more than change regimes; they also had a marked impact on the international order, changing alliances and creating new axes of conflict. As George Lawson (Anatomies of Revolution, 2019), among others, has observed, revolutions are inherently transnational in both their causes and effects. Revolutionary ideologies are imported and exported; revolutionary actors travel to other countries for preparation, training, and recruiting; and foreign countries intervene in revolutions and counterrevolutions as both supporters of old regimes and supporters or opponents of revolutionary leaders.

Chad Nelson’s book offers a valuable new take on how and why foreign countries respond to revolutions in other states. His theory is that countries respond aggressively to revolutions when they perceive a threat of contagion—that is, they fear that the revolution might serve as a model or inspiration to opposition movements in their own country. Note that this theory does not require that a revolutionary regime has an expansionary policy or actively tries to promote revolution abroad. All that is required is that a government faces its own domestic opponent who it sees as similar to the revolutionary actors in another, successful, revolutionary campaign. The insecurity of the former, then, is sufficient to lead its rulers to do all they can to combat the spread of contagion, including efforts to isolate, intimidate, attack (economically or militarily), and overturn the revolutionary regime—including making new alliances that might include former foes, if they share a fear of the same contagion.

What is unique about Nelson’s theory is that he explains the role of revolutionary ideology in precipitating international conflict not simply in terms of ideological differences, but through the role of a domestic opposition creating insecurity as responsible for the perception of threat. Thus, for example, Nelson argues that France was willing to help the Americans defeat Great Britain by supporting the American Revolution, even though the Americans were motivated by an anti-monarchical republican ideology, because in the 1770s the French court simply could not conceive of a republican revolution in France. Only after 1789 did the crowned heads of Europe unite against revolutions, seeing the threat from France spreading across the continent and reviving in 1830 and 1848. Similarly, the allies in World War I immediately took up hostilities against the communist revolution in Russia in 1918–21, even though at that time it presented no threat of outward aggression, while they did not take up arms against the fascist revolutions in Italy and Germany in the 1930s until the latter launched a war against them.

Nelson presents his theory clearly, but truly reinforces it with ten well-selected cases. To be clear, Nelson is not arguing that all revolutions produce fears of contagion, nor that revolutions must succeed for an international response to result. Rather, he claims that his theory does explain the international repercussions of most of the major revolutions in world history, and his cases are chosen to back up that contention. Thus he examines two early democratic revolutions, the American Revolution and the Dutch Patriot Revolution of the 1780s; the early nineteenth-century wave of revolutions in Spain, Portugal, Naples, Piedmont, and Greece; the Russian communist and Italian fascist revolutions of the early twentieth century; and the Iranian Islamic Revolution of 1979. In each case, Nelson examines the international context, the changes in the revolutionary state and its new ideology, and the reaction in other states, especially the major powers. Nelson’s case studies are thorough and well-informed. Most important, he is able to show that hostilities only arose where domestic oppositions stirred fears of contagion. Moreover, he shows that shifts in the international order were not only binary, affecting the revolutionary state and opponents as dyads; instead new international alignments were created in response to revolutions. Perhaps most striking was how the US, which had based its Middle East strategy in the 1970s on partnership with Iran and Israel, shifted after 1979 to becoming a major ally of Iraq in its war with Iran. Eventually, the desire to unite to contain Iran brought Israel closer to Arab states, including the UAE and Saudi Arabia, a shift that would have been unthinkable without the Iranian revolutionary threat.

To this reviewer, the value of Nelson’s thesis and insightful case studies is clear. In seeking to understand Russia’s invasions of Ukraine in 2014 and 2022, realists have argued that it was fear of NATO’s influence reaching to Russia’s borders that impelled Putin to act. But given that Latvia and Estonia have been NATO states that border Russia since 2004 without provoking hostilities, and NATO made it clear that there was no path for Ukraine becoming a member anytime soon, that explanation has always been problematic. I find more persuasive the argument that Putin, who has long had a deep fear of color revolutions and their spread to Russia, was motivated to invade Ukraine by his fear of the example that a successful, democratic, post-color-revolution Ukraine would offer on Russia’s doorstep.

Nelson’s book is thus an excellent addition to the literature on both revolutions and how revolutions impact international relations. It deserves a place on the bookshelves of scholars in both fields.