U.S. leaders are increasingly worried that China is gaining influence over traditional American allies, as the rising Communist power charts a more aggressive foreign policy course.

Those concerns are primed most recently by the new South Korean president’s decision to suspend deployment of a U.S. missile defense system, known as a THAAD. American and South Korean officials agreed in recent years that the THAAD is necessary protection against North Korea, but China fears that the system could undermine the power of its own nuclear arsenal. So they demanded an “immediate” halt to the deployment of the system, backed up by economic measures to hurt the South Korean economy.