MOON JAE-IN, who easily won South Korea’s presidential election Tuesday, was the beneficiary of another popular backlash against a ruling establishment perceived as corrupt and out of touch. Already unhappy with a slowing economy and shrinking opportunities for the young, South Koreans took to the streets by the hundreds of thousands last year when President Park Geun-hye was accused of conspiring with a friend to extort bribes from the country’s big conglomerates. After Ms. Park was impeached and jailed, Mr. Moon, a leftist former human rights lawyer who lost to her in the last presidential election, was the obvious alternative. His promises to boost government hiring, tighten regulation of the big companies and conduct a more modest and open presidency are a natural response to the mood of dissatisfaction.

Yet if Mr. Moon’s ascent can be described as a triumph for South Korea’s young democracy, it may pose a challenge to an already wobbly U.S. position in Asia. President Trump has made the denuclearization of North Korea a top priority of his new administration, pursuing — sometimes erratically — a strategy of sharply raising the pressure on the regime of Kim Jong Un while holding out the prospect of negotiations. Mr. Moon has advocated a more dovish approach — and he has expressed unhappiness with what looked like a U.S. race to put a new missile defense system, the Terminal High Altitude Area Defense (THAAD), in place before the election took place.

The hurried deployment of the THAAD batteries last month, literally in the middle of the night, “lacked democratic procedure,” Mr. Moon complained in an interview last week with The Post’s Anna Fifield and Yoonjung Seo. He has a point: Though the system is an important and needed counter to the growing missile threat from Pyongyang, the action looked like an attempt to circumvent Mr. Moon’s expressed reservations by creating a fait accompli.

Mr. Moon also sounded unhappy with Mr. Trump’s strategy of aggressively pursuing cooperation on North Korea with Chinese President Xi Jinping, saying South Korea should “take the lead on matters in the Korean Peninsula” rather than “take the back seat and watch discussions between the U.S. and China.” The new president has long been an advocate of rapprochement between the two Koreas and has advocated reopening two joint projects that provided North Korea with valuable streams of hard currency — a step that would run directly counter to Mr. Trump’s strategy of tightening sanctions.

It does not help that Mr. Trump recently trashed the “horrible” U.S.-South Korea free-trade agreement and suggested that South Korea should pay $1 billion for THAAD, in contravention of a bilateral agreement. That played into the hands of Mr. Moon, who has written that South Korea should “learn to say no” to Washington. Should Mr. Trump persist in that line, he could quickly sabotage both his North Korea initiative and bilateral relations with a vital American ally.

The good news is that Mr. Moon is for now striking a conciliatory note. He told Ms. Fifield that “President Trump is more reasonable than he is generally perceived” and that he agrees with the U.S. strategy of “applying sanctions and pressure” to Pyongyang if it leads to negotiations. This is a relationship that can be saved and even strengthened — if Mr. Trump handles it with care.