SEOUL, South Korea — Since his election in May, President Moon Jae-in of South Korea has been something of an odd man out in Washington’s “maximum pressure” campaign on North Korea, calling again and again for talks as President Trump was threatening “fire and fury.” At one point, Mr. Trump even accused him of “appeasement.”

But Mr. Moon’s persistence suddenly appears to be paying off.

Mr. Trump’s head-spinning decision to accept an invitation to meet with Kim Jong-un, North Korea’s leader, amounts to a remarkable diplomatic coup for Mr. Moon, who engineered the rapprochement in a whirlwind of diplomacy that began at the Winter Olympics last month and gained momentum faster than perhaps even he had anticipated.

Mr. Moon went out of his way to credit Mr. Trump with each breakthrough and personally appealed to Mr. Kim’s sister to engage in talks with the United States. After Mr. Kim played host to the South’s envoys in Pyongyang, the North’s capital, Mr. Moon immediately sent them to Washington to brief Mr. Trump, who agreed on the spot to an unprecedented meeting between a sitting American president and a North Korean leader.

Not only has Mr. Moon steered two headstrong, erratic adversaries away from a military conflict that could have been devastating for his nation, he has maneuvered the Trump administration into pursuing negotiations that it has long resisted — but that he and his allies on South Korea’s political left have long pressed for.