Presidents Biden and Yoon pledged to deepen and broaden the scope of the U.S.-South Korea alliance, but their success could depend on how China, Japan, and North Korea respond.

The Quad, composed of the United States, Australia, India, and Japan, is not a formal alliance. Still, the group has intensified its security and economic ties as tensions with China rise.

North Korea seeks to exploit major-power rivalries and weaken U.S.-South Korea ties with its ramped-up missile testing. Already, it has ratcheted up nuclear tensions on the Korean Peninsula to their …

President-Elect Yoon Suk-yeol has pledged to strengthen ties with the United States and take a more hard-line approach toward China and North Korea than his predecessor.