MANILA, Philippines — The United States, United Kingdom and a dozen other Western and Asian countries reasserted on Sunday that China’s expansive claims in the South China Sea are illegal based on a 2016 arbitration ruling. A joint statement issued by the nations said they rejected “destabilizing” actions in the disputed waters that threaten regional peace. The announcement commemorated a July 12, 2016, arbitration ruling by a tribunal established in The Hague under the United Nations Convention on the Law of the Sea, saying the landmark decision is “a significant milestone and is final, legally binding and definitive.” China refused to join the arbitration initiated by the Philippines in 2013 after a tense standoff in the contested waters a year earlier that ended with Beijing effectively seizing a disputed shoal. Beijing rejected the 2016 ruling and continues to defend its claims to virtually the entire sea passage, a key global trade route that has long been feared as one of Asia’ most active flashpoints. The areas has been the scene of repeated territorial standoffs inv