“There was a sense last year that President Trump at APEC was upstaged” by the Chinese president, said Robert Holleyman, who served as a deputy United States trade representative during the Obama administration. “This would’ve been an opportunity for a United States president to do a redo.”

Sue Mi Terry, a former C.I.A. analyst who is a senior fellow for Korea at the Center for Strategic and International Studies, said that Mr. Trump’s absence this year would be viewed as further evidence of what she called a “lack of interest” in cultivating American allies.

She said Mr. Trump’s murky approach to North Korea would be a “lost opportunity” in tending to relationships with leaders who already may have come to view him as an erratic and unreliable ally.