By David Brunnstrom, Josh Smith and Ju-min Park

WASHINGTON/SEOUL (Reuters) -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.

Much is riding on the meeting for Lee, who took office in June after a snap election called after his conservative predecessor - feted in Washington for his hard line on North Korea - was removed for attempting to impose martial law.

South Korea's economy relies heavily on the U.S., with Washington underwriting its security with troops and nuclear deterrence. Lee hopes to chart a balanced path of cooperation with the U.S., while not antagonising top trade partner China

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