The claim that international politics will be defined by a “clash of civilizations” has come back into the headlines. In April, Kiron Skinner, the head of the State Department’s Policy Planning staff, described U.S. relations with China in terms of a clash of civilizations. On Tuesday, national security adviser John Bolton reportedly remarked that relations between the United States and China have “elements of Samuel Huntington’s Clash of Civilizations.” These remarks have drawn pointed rebukes from Chinese writers, outlets and officials, including President Xi Jinping, lambasting the “clash of civilizations” as “dangerous” and “stupid.”

So what is the “clash of civilizations,” and why is it so controversial?

1. The “Clash of Civilizations” represented an attempt to understand the post-cold war world

Although the phrase “clash of civilizations” was first used in the context of world politics by scholar Bernard Lewis, most famously in a 1990 article in The Atlantic, the term is now strongly associated with a 1993 Foreign Affairs article by the political scientist Samuel Huntington. (Huntington later expanded the argument into a 1996 book, The Clash of Civilizations and the Remaking of World Order.)

Huntington argued that economic and political divisions would no longer be the source of conflict in the post-Cold War world. Instead, “the dominating source of conflict will be cultural” and “[t]he fault lines between civilizations will be the battle lines of the future.”

The article made a splash in part because it was so profoundly different from other arguments about the end of the Cold War. In 1989, for instance, Francis Fukuyama argued that the West faced no challenges to its blend of liberal democracy and capitalism. Huntington’s far more pessimistic vision of the future rejected that idea. His view was informed by the grimmer, bloodier end of Yugoslavia, which collapsed into civil war driven by ethnic and religious divisions.

Huntington defined a “civilization” as the highest grouping of cultural entities that shared fundamental traits. He claimed that although a German and an Italian village might be recognized as fitting into a common European home, no such deep ties bound European culture to Arab or Chinese cultures. Civilizational borders could encompass or divide existing political borders.

Huntington saw civilizations as leading to conflict partly because political boundaries did not match civilizational boundaries. That mismatch would yield conflict and division as peoples sought to redraw the map to fit civilizational boundaries.

Other conflicts would come about because of the jealousies and rivalries between civilizations. Civilizational differences would prove intractable and violent because they were “far more fundamental than differences among political ideologies and political regimes.” Huntington believed that such conflicts would especially lead to challenges to the West by non-Western civilizations because of the West’s long status as the dominant global culture.

2. The terrorist attacks of Sept. 11, 2001, brought it back to life

The “clash of civilizations” argument ignited a debate among academics and other commentators that lasted throughout the 1990s. Most of the academic commentary was strongly negative, even though the book became a bestseller.

Yet the real turning point in the argument’s popularity came with the terrorist attacks of Sept. 11, 2001. The U.S. media turned toward Huntington, and to Lewis and others, to provide theories to make sense of the new world.

In the aftermath of the attacks, many thought the “clash” thesis was prophetic. Huntington’s 1993 article followed Bernard Lewis in claiming that the key “clash of civilizations” would take place between Islam and the West, an argument he summarized in the controversial claim that “Islam has bloody borders.” Interest in the argument soared to new heights, even as critics reiterated and sharpened their attacks on the theory and the evidence on which it rested.

3. U.S. policymakers long resisted framing policy in civilizational terms

For decades, American policymakers repeatedly rejected the “clash of civilizations” thesis. In a 1998 speech at the United Nations, President Bill Clinton castigated the view that “there is an inevitable clash between Western civilization and Western values, and Islamic civilizations and values. I believe this view is terribly wrong.” After terrorist attacks in Paris, Secretary of State John F. Kerry argued that clashes between ISIS and the West were “not a clash of civilization” because “[t]hese terrorists have declared war against all civilization.” Finally, in 2016, President Obama told the Islamic Society of Baltimore that their example could “make clear that this is not a clash of civilizations between the West and Islam” but “a struggle between the peace-loving, overwhelming majority of Muslims around the world and a radical, tiny minority.”

Officials and commentators rejected the “clash” thesis because they feared that endorsing it would play into the hands of al-Qaeda and other violent extremist groups. In 2015, former secretary of state Hillary Clinton summarized the argument: “it helps to create this clash of civilizations that is actually a recruiting tool for ISIS and other radical jihadists who use this as a way of saying we’re in a war against the West. … If you are a Muslim, you must join us.” (Ironically, some experts now worry that “clash” rhetoric might mobilize right-wing Western extremists.)

4. Most academics reject the claim that a civilizational clash is inevitable

Despite official rejection of Huntington’s thesis, his argument remains widely assigned in university courses. According to the Open Syllabus Project, which collects and analyzes seven million English-language syllabi from more than 80 countries, the article and book are the 28th-most assigned text in their collection — more popular than Hamlet. It remains especially popular in political science courses, as well as in history and sociology.

Yet few instructors assign Huntington’s “clash” argument because they find it persuasive. Instead, instructors who assign it frequently do so to help students learn how to tear an argument apart.

The Cato Institute scholar Emma Ashford summarizes the academic work finding Huntington’s evidence for his position is weak. The strongest evidence against it is plain to see. The decades since Huntington’s argument was published have been more peaceful than the decades before. Data from the Uppsala Conflict Data Program shows that the highest numbers of battle-related deaths in recent years have taken place in Syria, Afghanistan, Iraq, and Yemen — none of which easily fit into Huntington’s vision. Similarly, despite the “bloody borders” line, most victims of terrorist attacks perpetrated by Muslims are themselves Muslim.

Even at a theoretical level, Huntington’s view of civilizations has been attacked as incoherent and reductive. The international-relations theorist Patrick Thaddeus Jackson points out that civilizations are not as fixed in their boundaries or as constant in their character as Huntington’s theory requires them to be. Rather, civilizations change over time, both internally and in their relations with each other. More succinctly, conflict is demonstrably not the only way civilizations can relate to each other.

Finally, an increasing number of academics also note the similarities between Huntington’s argument and earlier, overtly racist works, leading some to wonder if the argument merely dresses up prejudices in sophisticated verbiage.

Some academics worry, however, that the clash of civilizations might become a self-fulfilling theory. As Cornell University professor Steve Ward wrote in The Monkey Cage recently, if U.S. policymakers treat China as essentially and immutably different from the United States, the consequences could be dangerous.

5. The Trump administration has been more open to the “clash” thesis, but is inconsistent

Since President Trump won the 2016 election, observers have wondered whether the administration would pursue a policy deeply informed by the “clash” thesis. Yet the evidence that the Trump administration is pursuing a consistent “clash of civilizations” foreign policy is ambiguous.

The State Department has distanced itself from Skinner’s remarks, with one senior official declining to use the term and describing China instead as a “strategic competitor”. More concretely, President Trump himself has sought to improve relations with North Korea and has protected relations with Saudi Arabia, among others.

On the other hand, the administration’s hard line policies toward Iran and the president’s overt friendliness toward illiberal and culturally chauvinist leaders like Hungary’s Viktor Orban seem in keeping with “clash” thinking. The editor in chief of The American Conservative, frequently critical of Trump, sees Huntington’s thought as a major influence among nationalist conservatives backing the president.

And Trump has most convincingly followed the implications of Huntington’s theory in his immigration policy. That has been manifest in his preference for immigrants from culturally similar countries, his racist and nativist attacks on four Democratic congresswomen of color, his quest to ban travelers from several Muslim-majority countries, and his efforts to use legal procedures, enforcement actions, and a border wall to stem immigration from Latin American countries.

Nevertheless, these actions do not yet prove that Huntington’s decades-old writing are providing the core doctrine for the administration. Even if Huntington’s ideas resonate with many of Trump’s policies, Trump does not need to marry himself to them, or to any other intellectual orthodoxy.

Paul Musgrave (@profmusgrave, www.paulmusgrave.info) is assistant professor of political science at the University of Massachusetts Amherst