Alarmed over North Korea’s increasingly provocative behavior, the United States said Tuesday that it had started to deploy an antimissile system in South Korea that China has angrily opposed as a threat to its security.

The deployment of the Terminal High Altitude Area Defense system, or Thaad, came after North Korea launched four ballistic missiles on Monday, apparently in response to joint naval exercises by South Korea and the United States. Those launchings led South Korea to call for the accelerated deployment of Thaad.

A spokeswoman for the United States forces in South Korea said that one of five major components of the missile system had arrived on Monday. Officials said it could take a couple of months for the system to become fully operational. Defense Secretary Jim Mattis had urged the South Koreans to move ahead with the deployment of the system during a visit to Seoul in February. […]