Mr Moon Jae In is not Mr Donald Trump's kind of guy. The new president of South Korea is a former human rights lawyer, not a businessman. Mr Trump likes to threaten North Korea with isolation and aircraft carriers; Mr Moon is an advocate of dialogue and cooperation. The South Korean President is reserved, while the US President is bombastic.
These differences in style and policy will make for an awkward first summit, when the two leaders meet in Washington later this week. But it is crucial that they forge an understanding. The great danger for Mr Moon is that if he cannot persuade Mr Trump to see things his way, the US President's policy of "America First" could persuade him to launch a pre-emptive strike on North Korea's nuclear programme - eliminating a risk to US security, at the cost of massive retaliation aimed at South Korea.
Mr Trump has repeatedly insisted that North Korea will never be allowed to develop an intercontinental nuclear missile that could threaten the US. In public and private, he and his senior aides have insisted that the US will, as a last resort, use military action to counter the North Korean threat. The consequences would be horrendous. US Defence Secretary James Mattis recently predicted that it would lead to "a war more serious in terms of human suffering than anything we have seen since 1953... It will involve the massive shelling of an ally's capital".
Presidents Bill Clinton, George W. Bush and Barack Obama all concluded that the risk of the devastation of Seoul - a city of 10 million people - ruled out a strike on North Korea. But Mr Trump could come to a different conclusion. On a recent trip to the US, I met three people who have discussed North Korea with the President. One former official told me he believed that the Trump administration is bluffing and would never attack North Korea. Another believed there was a real possibility Mr Trump would choose military action, arguing that the President "doesn't think through consequences and he hates quiet waters". A third insider was certain that, as a last resort, Mr Trump would indeed authorise an attack on North Korea. His argument was: "There will be a point where homeland security trumps everything else."
Such thinking is surprisingly widespread in right-wing Washington. Senator Lindsey Graham, who is normally seen as a voice of reason in the Republican Party, has repeatedly insisted that the US cannot tolerate a North Korean nuclear threat. He has argued that while a war with North Korea would be "bad for the Korean peninsula... what it would not do is hit America".
Mr Trump has also debated the military options with foreign leaders. In a conversation with Philippine President Rodrigo Duterte, later leaked, Mr Trump asserted: "We can't let a madman with nuclear weapons on the loose like that. We have a lot of firepower, more than he has times 20, but we don't want to use it." He continued: "I hope China solves the problem. But if China doesn't do it, we'll do it."
That remark, which was made in April, looks more significant in the light of a tweet by Mr Trump last week, which read: "While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out."
The death of Mr Otto Warmbier, an American student who had been held prisoner in North Korea, has refocused US attention on the evils of the Kim Jong Un regime.
The change of leadership in South Korea last month complicates the Trump administration's options. President Moon's views on how to deal with North Korea seem closer to Beijing's than Washington's. Mr Moon has already begun to delay the deployment of the Terminal High Altitude Area Defence platform, an American anti-missile system that China vigorously objects to, but which the US believes is vital to the protection of US bases in South Korea. He is also a believer in the complete denuclearisation of the Korean peninsula and the resumption of economic cooperation with Pyongyang - both positions that are considerably more popular with Beijing than Washington.
Much now depends on whether Mr Moon can persuade the US President to moderate his approach to North Korea. It is possible Mr Trump's view that pressure from China has failed will give Mr Moon an opening to argue for a diplomatic offensive, aimed at achieving a freeze on Pyongyang's missile and nuclear testing.
One difficulty is that the US President has, so far, seemed most receptive to advice from strongman leaders like Mr Duterte. Mr Trump even asked the Filipino leader for his assessment of Mr Kim's mental stability, which is perhaps ironic given that Mr Duterte himself is famous for wild behaviour and once remarked that he would be "happy to slaughter" three million drug addicts.
Mr Moon is a rather different figure. His belief in human rights and dialogue might strike Mr Trump as "weak" or even "sad!", but the South Korean leader is not blind to the evils of North Korea, nor is he a wimp. His parents fled North Korea as refugees and he himself served in the South Korean special forces. He now has the heavy responsibility of persuading President Trump to give diplomacy another chance.