As the 2024 US presidential primary campaign season begins, the most likely final contest is a rematch between President Joe Biden and Donald Trump. Judging by the electoral map in 2020, Biden would be well-placed to win. But American politics is unpredictable, and any number of health, legal, or economic surprises could change the outlook. Hence, many foreign friends have been asking me what would happen to US foreign policy if Trump were to return to the White House.

The question is complicated by the fact that Trump himself is unpredictable. The presidency was his first political office, and his background translated into a highly unconventional political style. His success as a reality-television star meant that he was always focused on keeping the camera's attention - often with statements that were more outrageous than true, and by breaking conventional norms of behavior.

Trump also intuited that he could mobilize discontent by decrying the uneven economic effects of global trade and stoking resentment over immigration and cultural change, particularly among older white males without a college education. With a constant drip of populist, protectionist, and nationalistic statements, he earned himself equally constant media coverage.

Back in 2016, many expected Trump to move to the center to broaden his political appeal, as most normal politicians would do. Instead, he continued to play to his loyal base, which he used as a bludgeon against any congressional members of his party who dared to criticize or contradict him. Those Republicans who openly opposed him tended to lose their primaries to Trump-endorsed challengers. As a result, Trump has established almost complete control of the Republican Party. In the 2020 election, however, his appeal to the extreme right may have cost him the support of some moderate Republicans and independents in key swing states.

As president, Trump was different from all his predecessors. He often announced major new policies (or the firing of cabinet secretaries) on Twitter, and seemingly on a whim. His administration thus was characterized by frequent changes in top personnel and contradictory policy messages, with the president undercutting his own top officials. What he lost in organizational coherence, however, he made up for with his near-complete domination of the agenda. Unpredictability was one of Trump's most potent political tools.

Insofar as Trump has deeply held political views, they are eclectic, rather than traditionally Republican. He has long expressed protectionist opinions on trade and channeled nationalist resentment by claiming that America's allies are taking advantage of it. He has openly challenged the post-1945 consensus on the liberal international order and proclaimed NATO obsolete, leading John Bolton, one of his former national security advisers, to worry that he would withdraw the US from the alliance if re-elected. For his part, Trump recently promised to "finish the process we began under my administration of fundamentally re-evaluating NATO's purpose."

As president, Trump withdrew from the Paris climate agreement and abandoned the Trans-Pacific Partnership that President Barack Obama had negotiated. He weakened the World Trade Organization; imposed tariffs on steel and aluminum imports from allies; launched a trade war against China; withdrew from the Iran nuclear agreement; criticized the G7; and praised authoritarian leaders with well-known records of violating human rights. He was notably gentle in his relations with Russian President Vladimir Putin and skeptical of US support for Ukraine.

Polls show that American soft power declined considerably during the Trump years. Though tweets can help to set a global agenda, their tone and substance also can offend other countries. Trump paid very little attention to human rights, and his speeches paid short shrift to the principles of democracy that every president since Jimmy Carter and Ronald Reagan has espoused. Even critics who applauded Trump's tougher position on China faulted him for not working with allies in responding to Chinese behavior. Moreover, Trump undercut the advantages that America has long enjoyed as a leading influence within global institutions.

So, what would happen under a second Trump term?

Recall that before the 2016 election, 50 Republican former national security officials signed a statement warning that, "A President must be disciplined, control emotions, and act only after reflection and careful deliberation. ... Trump has none of these critical qualities. He does not encourage conflicting views. He lacks self-control and acts impetuously. He cannot tolerate personal criticism. He has alarmed our closest allies with his erratic behavior." When Trump won, these critics were excluded from any role in his administration, which would likely be the case again.

As a political leader set on aggregating power, Trump has clearly proved himself capable. But his temperament in governing has shown that he lacks the emotional intelligence that had underpinned the success of presidents such as Franklin D. Roosevelt and George H.W. Bush.

As Tony Schwartz, the writer of Trump's autobiography, once said, "Early on, I recognized that Trump's sense of self-worth is forever at risk. When he feels aggrieved, he reacts impulsively and defensively, constructing a self-justifying story that doesn't depend on facts. ... Trump simply didn't traffic in emotions or interest in others. ... A key part of the story is that facts are whatever Trump deems them to be on any given day." As president, Trump's personal needs often distorted his motives and interfered with his policy objectives.

Trump's temperament also limited his contextual intelligence. While his lack of experience in government and international affairs already made him less qualified than most of his predecessors, he then showed almost no interest in filling the gaps in his knowledge. Worse, his constant need for personal validation led to flawed policy choices that weakened American alliances - for example, after his summit meetings in 2018 with Putin and North Korea's Kim Jong-un.

Judging by Trump's behavior as an ex-president, nothing has changed. He remains unwilling to accept his 2020 defeat, and his campaign to return to the White House after next year's election has featured the extreme statements that mobilize his loyal base. If he succeeds, the only predictable feature of US foreign policy will be unpredictability.

From Project Syndicate