China’s leaders are again standing by North Korea after its nuclear test by blocking serious United Nations sanctions and refusing to cut off the flow of fuel, food and arms to Pyongyang. But this time Beijing may pay a strategic price for shielding its unsavory friends, as South Korea moves closer to deploying the U.S.-built Thaad defense system to protect itself from ballistic-missile attacks.

After publicly avoiding the topic for more than a year, President Park Geun-hye raised it last week in a national address. “Taking...