Relations between Seoul and Beijing continue to erode. South Korea revealed Dec. 29 that last month it had seized a Hong Kong-flagged tanker involved in an at-sea transfer of refined petroleum products to a North Korean vessel, a violation of sanctions against North Korea under U.N. Security Council Resolution 2375. The announcement comes as the United Nations is considering a list of 10 ships to sanction for illicit trade with North Korea, and follows the U.S. Treasury Department's release of images of Chinese vessels transferring petroleum products to North Korean ships at sea in contravention of U.N. sanctions. The incident will strengthen U.S. criticism of China's support for North Korea....