For a few months, everything seemed to be clicking for South Korean President Moon Jae-in as he pieced together crucial nuclear negotiations between the U.S. and North Korea following a year of intense animosity.

But he now enters a White House meeting with President Donald Trump with his status in the diplomatic driver's seat in doubt.

Pyongyang's surprise move last week to break off a high-level meeting with Seoul over U.S.-South Korean military drills while threatening to cancel next month's summit between North Korean leader Kim Jong Un and Trump cooled what had been an unusual flurry of diplomatic moves from the country after a provocative series of nuclear and missile tests.

It also underscored Seoul's delicate role as an intermediary between Washington and Pyongyang and raised questions about Moon's claim that Kim has genuine intent to deal away his nukes.

Seoul may lose much of its voice if Trump chooses to deal more directly with China, North Korea's only major ally, which is refusing to be sidelined in the global diplomatic push to resolve the nuclear standoff.

Seoul's presidential office said this week's meeting between Moon and Trump will be mainly focused on preparing Trump for his summit with Kim, scheduled for June 12 in Singapore.

A look ahead at the Moon-Trump meeting and the challenges they face with Kim:

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WHITE HOUSE TO SINGAPORE

Seoul insists Kim can be persuaded to abandon his nuclear facilities, materials and bombs in a verifiable and irreversible way in exchange for credible security and economic guarantees.

Moon and Trump will likely discuss potential steps that Trump can put on the table in Singapore. Their meeting at the White House may also include discussions on setting up three-way talks with Pyongyang or four-way talks also including Beijing to negotiate a formal end to the 1950-53 Korean War, which ended with an armistice, not a peace treaty.

Moon and Trump may exchange views on whether the allies should temporarily modify their joint military drills while engaging in denuclearization negotiations with North Korea, said Koh Yu-hwan, a North Korea expert at Seoul's Dongguk University and a policy adviser to Moon. There could also be deeper discussions on the future of the U.S.-South Korea alliance.

South Korea maintains an optimistic outlook for the Trump-Kim talks. Moon's foreign minister, Kang Kyung-wha, told South Korean lawmakers last week that the North has issued a commitment for "complete denuclearization." However, she said there's a "difference in opinions between the North and the United States over the methods to achieve denuclearization."

Officials in Washington have talked about a comprehensive one-shot deal where the North fully eliminates its nukes first and receives rewards later. But Kim, through two separate summits with Chinese leader Xi Jinping in March and May, has called for a phased and synchronized process where every action he takes is met with a reciprocal reward from the United States.

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THE DREADED "D'' WORD

Despite Seoul's reassurances, it remains unclear whether Kim will ever agree to fully relinquish his nukes, which he likely sees as his only guarantee of survival. For decades, North Korea has been pushing a concept of "denuclearization" that bears no resemblance to the American definition, vowing to pursue nuclear development unless Washington removes its troops from South Korea and the nuclear umbrella defending South Korea and Japan.

Kim declared his nuclear force as complete in November, following the country's third flight test of an intercontinental ballistic missile. At a ruling party meeting in April, the North announced that it was suspending all nuclear and ICBM tests and will close its nuclear testing ground because its mission had come "to an end."

The announcements were clearly designed to communicate that Kim sees himself as entering the negotiations with Trump from a position of strength and expects to be treated as a leader of a full-fledged nuclear state.

"The success of the Trump-Kim meeting will be determined by whether it turns out to be a denuclearization negotiation or an arms reduction negotiation between two nuclear states," said Du Hyeogn Cha, a visiting scholar at Seoul's Asan Institute for Policy Studies. "So far, the North has built conditions for the meeting to become the latter."

Trump may find it difficult to politically justify a deal with North Korea that's significantly less than a "complete, verifiable, irreversible denuclearization" when he just abandoned a major nuclear deal with Iran that he claimed was too weak.

But it's not clear whether he can coax a stronger deal from North Korea than the one Barack Obama got from Iran. Iran's nuclear program is nascent and its weapons capability theoretical. North Korea's arsenal now includes purported thermonuclear warheads and developmental ICBMs potentially capable of striking the continental United States.

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CHAMPAGNE POPPED TOO EARLY?

While Seoul can be credited for coordinating the diplomatic approach toward North Korea, South Korean officials may have been too optimistic about the signs they were seeing from Pyongyang.

During a March visit to the White House where Trump agreed to a summit with Kim, Moon's national security director, Chung Eui-yong, said Kim told visiting South Korean officials in Pyongyang that he "understands" that the joint military exercises between the allies "must continue." This was then seen as an important departure from the past, when the North thoroughly rejected the drills.

But Kim has yet to deliver any similar comment on record. In lashing out last week against the U.S.-South Korean drills and Trump's hard-line national security adviser, John Bolton, North Korea used familiar language to justify its nuclear weapons, saying the United States must eliminate its "hostile policy and nuclear threats and blackmail" against the North as a "precondition for denuclearization."

It's possible that Chinese President Xi persuaded Kim to adopt a tougher stance over U.S.-South Korean drills during their summits, Cha said.

Kim might have asked China to soften its enforcement of sanctions aimed at the North. He also might have sought Chinese commitments to strongly oppose any military measure the United States might take should his talks with Trump fall apart and the North starts testing missiles again.

"It's hard to say there are clear signs that the North is employing a meaningfully different approach on denuclearization," Cha said.

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Follow Kim Tong-hyung on Twitter at @KimTongHyung.