"There's just a lot of anxiety growing in South Korea about where the alliance is headed," said Jenny Town, the assistant director of the U.S.-Korea Institute in Washington and managing editor at 38 North, a website that provides analysis on North Korea. "A lot of South Koreans just feel like they're not part even of the deliberations and as an alliance they are not consulted."
Concerns about potential U.S. military action against Pyongyang increased in September after Trump hinted of force in a tweet. The president also recently said that Secretary of State Rex Tillerson was "wasting his time" on diplomacy.
The world's largest economy has also been flexing its military might in recent weeks: the Navy has three carrier strike groups operating around Asian waters while B-1B bombers have been conducting exercises over the Korea Peninsula.
Moon, who has been pushing for dialogue with Pyongyang, has repeatedly said he wants no military action without Seoul's approval.
"The thing that has been sucking the wind out of the room of North Korean discussion has been, will the U.S. conduct a preventive attack" said Bruce Klingner, former chief of the CIA's Korea branch and now senior research fellow for Northeast Asia at the Heritage Foundation's Asian Studies Center.
According to Klingner, another concern is that the Trump administration will abandon Seoul once North Korean leader Kim Jong Un shows the ability to hit the U.S. with an intercontinental ballistic missile.