North Korea tested a nuclear device earlier this month in defiance of a U.N. ban on such actions.
In response, the U.N. Security Council imposed sanctions on the North Korean government. Under the U.N. charter, all member states must comply with decisions of the Security Council.
Due to its relationship with North Korea, it is believed that China will “secretly” ignore the sanctions imposed by the U.N. and continue to trade with North Korea. Therefore China will cause the sanctions to be completely ineffective. (China is the “weak link” in the U.N. sanctions.)