Moreover, staged disarmament could leave South Korea and Japan "exposed to the North's shorter-range missiles" with the rogue state's threat likely boosted by a stronger domestic economy, the note continued.
"Any agreement that removes the North Korean nuclear threat from the United States, but leaves Japan and South Korea vulnerable to a North Korean attack would be cause for concern," echoed Troy Stangarone, senior director at Korea Economic Institute, a Washington-based think tank.
There's some hope among Japan, Taiwan and certain Southeast Asian nations that a reduction of American forces in South Korea could result in a reordering of U.S. deployments to better counter China's maritime assertiveness, the IISS note said. But there's "no guarantee" of that happening, it continued.
Rather, a troop reduction would "send a signal to the rest of the region that the United States was a waning presence rather than a source of stability," said Stangarone.
Earlier this month, American Defense Secretary James Mattis said U.S. troops wouldn't be part of initial negotiations with Kim's government even though that has long remained a North Korean prerequisite for peace.
During the 1970s, the U.S. withdrew some forces in South Korea in a move that prompted Seoul to develop national defense capabilities that included a brief clandestine nuclear weapons program. "We would certainly want to avoid that outcome," warned Squassoni.