South Korean President Yoon Suk Yeol set off on Monday for the for the United States and a summit with President Joe Biden at a time of rare questioning in South Korea of an alliance that has guaranteed its security for decades. Yoon's April 24-29 trip is the first state visit to the U.S. by a South Korean leader in 12 years and will mark the 70th anniversary of a partnership that has helped anchor U.S. strategy in Asia and provided a foundation for South Korea's emergence as an economic powerhouse.