In the better part of a decade that the United States has needed to negotiate, ponder, renegotiate, fight over, ponder some more and finally ratify a free-trade agreement with South Korea, a lot has changed in this neighborhood.

The U.S. share of Korea’s import market has been nearly halved, from 16 percent to 9 percent; the United States now trails China, the European Union and Japan in trading with Korea, the world’s 12th-biggest economy. China is the principal beneficiary of Korean investment. Asian countries have signed or are negotiating nearly 300 trade liberalization treaties among themselves, with the United States party to precisely none of them, according to a report last month from the Council on Foreign Relations. In the past decade, China and India have doubled in wealth, and China’s military has grown commensurately.

This week’s expected ratification provides a suitable occasion to celebrate the U.S.-South Korean alliance, as President Lee Myung-bak told me and The Post’s Chico Harlan in an interview Monday. Korea’s evolution from mendicant to donor nation, working alongside the United States in Afghanistan and elsewhere, provides the best possible argument for nurturing such alliances.

But the occasion also highlights the fragile status of U.S. leadership in this dynamic region. The time it took for the United States finally to say yes to something so clearly in its own interest is disheartening; that there’s nothing remotely comparable on the horizon is doubly so.

Lee will meet with President Obama and address a joint session of Congress Thursday; the two leaders will then take a victory lap in Detroit, which hopes to sell more cars to Korea thanks to the trade agreement. During the interview in the presidential Blue House Monday, Lee made clear that he considers the pact important for the jobs it will create in both countries but even more for the message that it will send of U.S. “reengagement” in the region.

And why does that matter? The conversation invariably comes back to rising China.

It is “understandable and unavoidable,” Lee said, that Asian countries are developing ever closer economic ties with China. “At the same time, many countries worry about sustainable security, peace, values such as democracy. . . . U.S. reengagement is crucial for that.”

Lee said Asian nations want the United States to get along with China but also to counterbalance China. Given China’s territorial claims, and their long memories of imperial dominance, “they do feel threatened by China to a certain extent.”

A senior U.S. official told me that the administration understands those anxieties and has responded to them, working on ties with democracies from India to Australia to Korea while shaping a pragmatic working relationship with China.

But leaders in the region also take note of Obama’s call to refocus on nation-building at home, while some Republican candidates draw cheers for even starker isolationism. Viewing U.S. military presence as essential for stability, they watch nervously as the U.S. shipbuilding budget and other military accounts are threatened. And they notice that, beyond advocating for a regional trading alliance that has yet to gain traction, the United States has been a wallflower as more and more trading partners take to the floor.

“Today, the United States lacks an ambitious trade policy,” the Council on Foreign Relations report said.

The report is no pre-election assault; it is the product of a bipartisan commission led by Democrat Tom Daschle, former Senate majority leader and a first-circle Obama fan, and Republican Andrew Card, President Bush’s former chief of staff.

“For too long the United States has been on the sidelines as other countries have found new ways to deepen their commercial relationships,” the commission concluded. “U.S. trade policy lacks both direction and momentum. . .

Lee, ever the polite and supportive ally, said it is only “natural” that the United States is “readjusting its military budget” after a decade of wars in Iraq and Afghanistan. But he said he is confident that when it comes to the Asia-Pacific region, U.S. leaders “will make the right choice.”

“As they reduce the budget for one area, I’m sure the budget for another area will correspondingly increase, commensurate with its importance,” Lee said. A bit later he added: “We do find solace that American politics has a rich history, in times of grave challenge, of politicians, no matter which party they come from, tending to rally around their leader.”

The idea sounds almost quaint. Here’s hoping members of Congress are paying attention when Lee brings his message to their house.