For millennia, China has used the Korean Peninsula as a defensive barrier on its eastern flank. At the same time, the Koreas have been a constant source of security concern for its northern neighbor. Over the weekend, China's reactions to developments on either end of the peninsula reflected its complicated relationship with the two Koreas. On Sunday, in response to the latest demonstration from North Korea of its nuclear weapons program development , Beijing imposed a ban on imports of the country's coal through the remainder of 2017. The stinging rebuke to Pyongyang, if fully implemented, will cut the cash-strapped country's export revenue by a third. Hours later, China issued a direct warning to the South Korean conglomerate Lotte , which is nearing completion of a deal with Seoul that is key to the deployment of the U.S. Terminal High-Altitude Area Defense (THAAD) anti-ballistic missile system.