It was here that big, peaceful candlelit protests against Mr Moon's disgraced predecessor, Park Geun-hye.Park began late previous year, helping pave the way for his election win months later. A couple of days later national security adviser H.R. McMaster assured his South Korean counterpart that the US would pay for the system. The system has angered Beijing, which says its powerful radars allow Washington to spy on its own military operations. Even though he did not go to Pyongyang himself, Im was convicted and jailed over the trip.
Mr Moon, 64, said: "I will urgently try to solve the security crisis".
Moon came into office on Wednesday following his landslide victory in a rare presidential by-election the day before.
The deployment of THAAD in South Korea was really a United States strategic move against China and Beijing's efforts would be required to convince Trump to remove it, Boyle recalled.
Moon was part of the liberal Roh Moo-hyun administration involved in the last inter-Korean summit of 2007, although almost a decade of conservative rule in Seoul has seen ties sour to the point of zero cooperation amid persistent North Korean nuclear and missile tests.
Moon expressed his hope to visit Trump and Xi in the near future, pledging cooperation with both in denuclearizing North Korea.
Taking the oath of office in Seoul on Wednesday, Moon said he will further bolster the alliance and "immediately fly to Washington if necessary" for the sake of peace on the Korean Peninsula.
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Just days before the election, in an interview with the Post's Anna Fifield, Moon said "the alliance between the two nations is the most important foundation for [South Korea's] diplomacy and national security".
Moon waved to well-wishers through the sunroof of his limousine, which was flanked by police motorbikes.
The White House congratulated Moon, saying it looked forward to working with him to strengthen the longstanding U.S.
At his first news conference at the presidential Blue House, Moon introduced his nominees for prime minister, the country's spy chief and his presidential chief of staff.
Also yesterday, Chinese President Xi Jinping called Moon, in which the South Korean President said he wants to work closely with China to denuclearize the Korean peninsula, including restarting six-party talks with world powers, "and other methods, as soon as possible".
Moon's election could add volatility to relations with Washington, given his questioning of the THAAD deployment, but it was not expected to change the alliance significantly, a USA official said.