SEOUL, South Korea — With diplomacy moving apace to end North Korea’s nuclear weapons program, Northeast Asia is bracing for something few had thought likely just months ago: a reduction or withdrawal of American troops from South Korea.

These forces have been the bedrock of the 65-year-old alliance between Seoul and Washington since the 1950-53 Korean War, serving as a bulwark against North Korean aggression and preserving a shaky peace that allowed South Korea to build its economy into a global powerhouse.

Now their presence is being questioned by President Trump, who is skeptical of maintaining a costly American military presence overseas, and by North Korea’s leader, Kim Jong-un, who called last week for a peace treaty formally ending the Korean War.

As Mr. Trump and Mr. Kim prepare to meet within the next few weeks to discuss peace and an end to North Korea’s nuclear program, Mr. Trump has ordered the Pentagon to prepare options for reducing the number of American troops in the South, according to several people briefed on the deliberations.