When Chinese and American fighter jets squared off over the Yellow Sea earlier this year, Seoul’s defense minister protested not to Beijing but to the commander of U.S. Forces Korea. It was a small sign of a larger shift: a treaty ally increasingly sensitive to Beijing’s concerns.

That matters when American and South Korean defense chiefs meet at the Security Consultative Meeting this fall. Seoul wants a deadline for assuming wartime operational control of allied forces, with President Lee Jae Myung insisting the transfer occur before his term ends in 2030.