President Lee Jae-myung of South Korea meets President Trump at Washington Monday with North Korea at the top of the agenda while North Korea heaps  scorn on Seoul’s “appeasement offensive.”

Trashing the notion of Mr. Lee working with Mr. Trump on resuming dialogue with North Korea’s leader, Kim Jong-un’s younger sister, Kim Yo-jong, told North Korean officials not to “forget that anyone in the Republic of Korea is the top-class faithful dog of the United States.” No way, she said, can South Korea “be a diplomatic partner” of the Korean regime,

Ms. Kim’s blast at South Korea, carried in English by Pyongyang’s Korean Central News Agency, was the latest North Korean attack on South Korea’s fledgling liberal leadership as it works to convince the North Koreans of its goodwill. One sign of Mr. Lee’s soft-line policy: postponement of half the annual military exercises that South Korean and American forces began this week.

Ms. Kim, whose colorful diatribes reflect what her brother wants said but not under his own name, claimed South Korea had “a new combined operation plan” for stripping the North of its “nuclear and missile capabilities” and attacking the North. “We have witnessed and experienced the dirty political system of the Republic of Korea for decades,” KCNA quoted her as saying, “and we are sick and tired of it.”

That outburst contributed to worries in South Korea that Mr. Lee may get nowhere in trying to convince Mr. Trump of the importance of the Korean American alliance — or the dangers inherent in the North’s much strengthened alliance with Russia as seen in its contribution of troops and arms for Russia’s war in Ukraine.

Mr. Trump’s “indifference to NK-Russia military partnership unnerves South Korea,” the Korea Times said, calling for “assurances of a coordinated response to North Korea’s military cooperation with Russia, which may outlast the ongoing war.” The inference was that North Korea, with Russian support, might attack South Korea in return for the North’s support in Ukraine. 

The  prospect of Mr. Trump hosting the left-leaning Mr. Lee at the White House has also raised concerns that Mr. Lee may be so eager for reconciliation with North Korea that he will not object to scaling down the American military commitment to South Korea. Right now 28,500 American troops are in the country.