The first debate between the Republican Party’s candidates for next year’s US presidential election revealed major schisms over foreign policy. While former US Vice President Mike Pence and former US ambassador to the United Nations Nikki Haley defended America’s support for Ukraine in Russia’s war of aggression, Florida Governor Ron DeSantis and businessman Vivek Ramaswamy expressed scepticism. Former president Donald Trump—the unquestioned frontrunner—skipped the event, but he, too, has objected to US involvement in that conflict.

Polls show that rank-and-file Republicans are as divided as the candidates. That raises concerns that if an isolationist Republican wins in 2024, it could mark a turning point for the US-dominated international order established at the end of World War II.

Historically, American public opinion has oscillated between extroversion and retrenchment. Having witnessed the tragic consequences of the isolationism of the 1930s, President Franklin D. Roosevelt launched the process that culminated in the creation of the Bretton Woods institutions in 1944 and the United Nations in 1945. President Harry Truman’s post-war decisions then led to permanent alliances and a continual US military presence abroad. The United States invested heavily in European reconstruction through the Marshall Plan in 1948, created NATO in 1949 and led the UN coalition that fought in Korea in 1950.

These actions were part of a realist strategy to contain Soviet power. But containment was interpreted in various ways, and Americans later had bitter, often partisan debates over interventions in developing countries like Vietnam and Iraq. Still, while the ethics of intervention were called into question, the value of sustaining a liberal institutional order was much less controversial. As the American theologian Reinhold Niebuhr once observed, the ‘fortunate vagueness’ of liberal internationalism had saved it from succumbing to ideological rigidity.

The liberal international order thus enjoyed broad support in US foreign-policy circles for decades after World War II. But in the 2016 presidential election, Trump’s argument that the post-1945 alliances and institutions had benefited others at America’s expense resonated strongly with many voters. To be sure, his populist appeal rested on more than an attack on US foreign policy. He also tapped into widespread anger over the economic dislocations caused by globalisation and the post-2008 recession, and exploited polarising cultural changes related to race, the role of women and gender identity. But by blaming economic problems on ‘bad trade deals with countries like Mexico and China and on immigrants competing for jobs’, Trump successfully linked nativist resentment to US foreign policy.

Of course, Trump wasn’t the first to apply this formula. The populist response had antecedents in the 1920s and 1930s. More than 15 million immigrants had come to the US during the first two decades of the century, sowing fears among many white Americans that they were being overwhelmed. In the early 1920s, a resurgent Ku Klux Klan helped pushed through the National Origins Act to ‘prevent the Nordic race from being swamped’ and to preserve the older, more homogeneous America. Similarly, Trump’s election in 2016 reflected, rather than caused, the deep racial, ideological and cultural rifts that had been developing since the 1960s.

While many analysts worry that American retrenchment could result in the kind of international disorder that plagued the 1930s, Trump supporters argue that his administration’s less generous and tougher stance produced greater stability abroad and support at home. Whatever the case, Trump’s election represented a clear shift away from the liberal tradition.

Some believe that Trump’s rise was caused by the failure of liberal elites to reflect the underlying preferences of the American people. But that is facile. Of course, there are many strands of American public opinion, and elite groups are generally more interested in foreign policy than the public at large. Nonetheless, we do have a good sense of where the public has stood over time.

Since 1974, the Chicago Council on Global Affairs has surveyed Americans on whether it is better to play an active global role or to stay out of world affairs. Over that period, roughly one-third of the public, hearkening back to the 19th-century tradition, has been consistently isolationist. That number reached 41% in 2014; but, contrary to popular myth, 2016 was not a high point of post-1945 isolationism. At the time of the election, 64% of Americans said they favoured active involvement in world affairs, and that number rose to 70% in 2018—the highest recorded level since 2002.

Although full-scale 1930s-style isolationism is highly unlikely, many analysts still worry that a failure to support Ukraine could signal a return to American retrenchment, auguring a serious weakening of the international order. Russian President Vladimir Putin’s invasion was a blatant violation of the UN charter. If Russia prevails in occupying Ukrainian territory, it will have undercut the liberal principle prohibiting the use of force to alter a country’s borders. The solidarity among NATO countries in applying sanctions and supplying military equipment to Ukraine thus is not only moral, but also practical and realistic.

The outcome in Ukraine will have serious implications for the future of Europe and the wider world. Although Putin and Chinese President Xi Jinping entered a ‘no limits’ partnership just before the invasion, China has been cautious, so far, in offering material support to Russia. Chinese leaders are doubtless concerned about Putin’s risk-taking, and worried that the alliance is proving too costly to Chinese soft power. If Putin prevails, however, China may conclude that taking such risks pays off—a lesson that will not have been lost on the rest of the world, either.

Those arguing that America doesn’t have an important national interest in helping Ukraine are wearing historical blinders. Their naivety (if not bad faith) should disqualify them from seeking the presidency.