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To make matters worse, Trump once asked Seoul via a tweet to pay the $1 billion bill for the anti-ballistic missile system. In order to address this awkward situation, the Moon administration changed the course and ordered a thorough environmental assessment of THAAD and halted installation of four launchers for the THAAD battery that have already arrived in South Korea.

The changes certainly helped to assuage Beijing but agitated Washington, which believes that the THAAD deployment will be effectively delayed. As Moon seeks to consolidate South Korea's alliance with the US, explaining his government's moves on the THAAD issue to Trump will be a hard sell.

On the economic front, the talks between Moon and Trump may be less controversial. Moon's entourage includes a large number of businessmen, who may bring investment or jobs to the US, a welcome move for Trump.

Nonetheless, as Trump has made it clear that the US-Korea Free Trade Agreement (FTA), which came into effect in 2012, should be renegotiated, South Korea is under pressure to make more concessions in any future FTA talks.

Moon is trying to jump start South Korea's economy, which has been long troubled by a chronic downturn, so Moon cannot avoid exchanging views with Trump over the FTA issue.

Moon's trip will not be easy. He goes to Washington with many questions in his mind. He and Trump will struggle to reconcile their policy priorities and forge a consensus over some thorny security and economic issues.

Moon tried to lower people's expectations of his trip, claiming that he does not seek concrete results on specific issues from his visit. Certainly, Moon will not be able to reap impressive results from this visit, but he may try to put the shaky start of Korea-US relations on a solid foundation and prevent their policy differences from undermining their alliance.

The author is an associate professor at the School of International Studies and deputy director of the Center for International Strategic Studies, Renmin University of China. He is also a senior research fellow with Pangoal Institute. opinion