President Yoon Suk-yeol charmed Washington last week on his first state visit to the US.

His speech before a joint session of Congress was received warmly and his rendition of “American Pie” was the highlight of a lavish state dinner.

He relished the hospitality, much as President Joe Biden did on a visit to Ireland in mid-April.

The visit will be remembered, however, as a turning point, not in relations between South Korea and the US but in relations between South Korea and China.

President Yoon aligned South Korea firmly with the US in the competition with China. This position reverses the stance of former President Moon Jae-in who focused on engagement with North Korea while trying to keep a low profile in the growing tension between the US and China.

Though Yoon and Biden come from different times and places, their worldviews reflect the “Pax Americana” mindset that has dominated strategic thinking in both countries since the Korean War.

This mindset holds that the US is a leader and defender of the “free world” through a network of alliances.

Adherents to this mindset argue that the US and its allies derive mutual benefit from the relationship that justifies the sacrifices that both sides make.

As the leader, the US bears the cost of maintaining the world’s most powerful military while allies sacrifice varying degrees of national independence.

“Pax Americana” has worked, not just for South Korea, but also other allies in Asia and Europe. None of the nations tied to the US with a defense treaty has been attacked since the treaties went into effect.

Through openness and free trade, the US and its allies have developed deep economic and cultural ties that have created broad prosperity on all sides.

No nation is a better example of this than South Korea.

Broad and sustained public support in the US and its allies kept “Pax Americana” together, particularly during the Cold War.

With the end of the Cold War, however, support began to weaken as nations developed new interests.

One of those interests was economic engagement with a booming China. Another was European integration and the subsequent expansion of the EU.

During this period, South Korea developed deep economic ties with China and growing ties with other Asian nations, such as India and Vietnam.

Donald Trump’s rise to the presidency in 2016 greatly weakened “Pax Americana” as he bashed US allies, including South Korea, and threatened to withdraw the US from alliances.

Things changed in 2021 as Joe Biden started his term by trying to revive “Pax Americana,” but the US public remained focused on other issues, and allies wisely remained skeptical.

Russia’s invasion of Ukraine in February 2022 brought the European wing of “Pax Americana” closer than it had been since last years of the Cold War in 1980s.

Meanwhile, an increasingly bellicose China and North Korea brought the Asian wing together.

President Yoon’s rapprochement with Japan and shift toward the US reflect these changes.

The big question for the future is the sustainability of “Pax Americana.” In 2022, the US public rallied around Ukraine and US support for it, but enthusiasm has waned as the war has dragged on.

Support has weakened most among Republicans and independent voters. Likewise, US support for Taiwan remains strong, but only 37 percent believe the US should offer military support if China invades. Support for Taiwan was lowest among independents and strongest among Democrats.

Traditionally, Republicans have been stronger supporters of “Pax Americana” than Democrats, but over the past decade, the parties have seen sharp shifts in their base of support. Blue collar voters have moved to the Republicans while college educated voters have moved to the Democrats.

In the process, Republicans have become more populist and Democrats more elitist.

Weakening support for “Pax Americana” among Republicans and independents is a bad sign because together, they can elect a president and take control of Congress.

Should that happen, “Pax Americana” will face an emboldened challenge from isolationism, leaving US allies out in the cold again.

In an interview on CBS News “60 Minutes” in December 2022, French President Emmanuel Macron argued that France should not be dependent on the US.

He said, “Just imagine on your side, would you accept as a US citizen to say, ‘My security and my future will depend on an election in France?’ No, I cannot imagine.” As the 2024 US presidential election approaches, South Korean leaders should heed this warning from France, America’s oldest ally.

(Robert J. Fouser is former Associate Professor of Korean language education at Seoul National University.)