South Korea's new president, Lee Jae Myung, will face a pivotal moment on Monday when he meets U.S. President Donald Trump in Washington for their first summit, as the countries' decades-old alliance strains to confront rapid geopolitical changes.

President Lee Jae Myung, left, and first lady Kim Hea Kyung arrive at Joint Base Andrews in Maryland on Aug. 24 from Tokyo to begin a three-day trip to the United States for a bilateral summit with U.

President Lee Jae Myung (left) and Japanese Prime Minister Shigeru Ishiba on Aug. 23 announce their joint declaration at a bilateral news conference following their summit at the latter's residence in Tokyo.