SINGAPORE — Embarking on a self-described “mission of peace,” President Donald Trump's seat-of-the-pants foreign policy is facing its toughest test yet as he attempts this week to personally broker an end to North Korea's nuclear program with Kim Jong Un.

The impulsive American president, who just days earlier called into doubt a pillar of the Western alliance, is set to face his match on the global stage as he prepares to meet Kim in Singapore. In the historic first meeting between the leaders of the technically-still-warring nations, Trump is prioritizing instinct over planning. Unlike traditional summits between heads of state, where most of the work is completed in advance of a photo-op, U.S. officials say the only thing certain ahead of these talks will be their unpredictability.

Ever since Trump shocked allies, White House officials, and, by some accounts, the North Koreans themselves when he accepted Kim's March invitation for a meeting, the two leaders have lurched toward an uncertain encounter that may well determine the fate of millions.

“It's unknown territory in the truest sense, but I really feel confident,” Trump told reporters Saturday. “I feel that Kim Jong Un wants to do something great for his people and he has that opportunity and he won't have that opportunity again.”

Trump's engagement with Kim fulfills the North Korean ruling family's long-unrequited yearning for international legitimacy, itself a substantial concession that could weaken more than a generation of U.S. efforts to isolate the country on the global stage.

“It's never been done before,” Trump said Saturday. “And obviously, what has been done before hasn't worked.”

A triumvirate of forces is bringing the meeting to fruition, said Scott Snyder, Senior Fellow for Korea Studies and Director of the Program on U.S.-Korea Policy at the Council on Foreign Relations, describing the summit as “Produced by Kim, directed by (South Korean President Moon Jae-in), and inspired by Trump.”

The hard-hitting sanctions and desire for legitimacy brought the youthful Kim to the table. The South Korean leader's efforts to avert a potentially catastrophic U.S. first strike pushed both leaders to take a risk. And Trump is the first U.S. president willing to sit-down with the autocratic ruler with so few concessions, believing his self-professed negotiating prowess will guide him though uncharted diplomatic waters.

Raising expectations in advance of the meeting, Trump said the outcome will rely heavily on his own instincts. The U.S. president, who prides himself on his deal-making prowess, said he will know “within the first minute” of meeting Kim whether the North Korean leader is serious about the nuclear negotiations.

White House aides described Trump in the days after receiving the initial Kim invitation as being obsessed by visions of winning the Nobel Peace Prize and using “The Art of the Deal” to put his mark on the global order. In recent weeks Trump's enthusiasm has been tempered somewhat by the challenge of deal-making with such an unpredictable opponent. And there are worries from the White House to East Asian allies that Trump's desire for an agreement will lead him to accept any deal — even if it's a bad one.