As tensions over Pyongyang's weapons programme have soared, the US president has traded personal insults and threats of war with North Korean leader Kim Jong-Un, but the South's capital and its 10 million inhabitants would be on the front line of any conflict.
"Getting ready to leave for South Korea and meetings with President Moon, a fine gentleman," Trump tweeted early today from Japan, the first stop on his Asian tour, adding: "We will figure it all out!"
Trump walked down the steps of Air Force One at Osan air base outside Seoul accompanied by the First Lady, and was welcomed by South Korean foreign minister Kang Kyung-Wha.
The US president flew in from Japan after securing Tokyo's full support for Washington's stance that "all options are on the table" regarding Pyongyang, and declaring its nuclear ambitions "a threat to the civilised world and international peace and stability".
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Trump's relationship with Moon, a former human rights lawyer, is noticeably cooler, stoking concerns about the decades-old alliance and fears in Seoul that it could be sidelined by the US in favour of Tokyo.
South Korea is rolling out the red carpet for Trump as it seeks messages of assurance about the alliance and US resolve.
At the same time, while Trump has threatened Pyongyang with "fire and fury", Moon is mindful that much of Seoul is within range of the North's artillery and in an address to parliament last week demanded: "There should be no military action on the peninsula without our prior consent."
"However calm Koreans are reported to be about the Trump-Kim war of words, we cherish our lives as much as Americans do theirs and feel scared by the outlook of war," it added.
Kim Hyun-Wook, a professor at the Korea National Diplomatic Academy, told AFP that the two allies have "subtle differences in their positions" and underlying suspicions about each other.
Citizens views are mixed, with both "No Trump" and "Welcome President Trump" demonstrations taking place in downtown Seoul since the weekend, sometimes only a few blocks apart.
Another issue will be trade between the two nations, with Trump expected to bring up the renegotiation of the five- year-old US-Korea free trade agreement which he has called a "horrible deal" and a "job killer".
His first stop in the South will be Camp Humphreys, where US forces stationed in the country have moved their headquarters from downtown Seoul.
Tomorrow, Trump will speak to South Korean MPs but his visit will not include a trip to the Demilitarised Zone dividing the Korean peninsula, with the administration downplaying the destination as "a little bit of a cliche".
Some observers have fretted that a gaffe by a president given to off-the-cuff remarks could send tensions rising on the peninsula.
"If Trump says anything that can provoke North Korea, it could send military tensions soaring again," said professor Koo Kab-Woo from the University of North Korean Studies in Seoul.