SEOUL, South Korea — He won the presidency promising a shift toward dialogue with North Korea. He argued that sanctions and pressure alone would never persuade the North to scrap its nuclear arsenal. And he pledged to “say no to the Americans if necessary.”

But six months after South Korea’s president, Moon Jae-in, returned the nation’s liberals to power, his plans to ease tensions on the Korean Peninsula have gone nowhere.

President Trump has doubled down on sanctions in the standoff with the North, dismissed talks as a waste of time, stepped up military drills and rattled the region with pugnacious threats. And he has barely disguised his disagreement with Mr. Moon, openly accusing South Korea, an ally of 67 years, of “appeasement.”

Now, as Mr. Moon prepares to host Mr. Trump’s first visit to South Korea, he finds himself caught in a bind, compelled by the White House and conservatives at home to support hard-line policies that he once criticized — and that many of his supporters worry are dragging their nation to the brink of war.