People pay tribute during a memorial ceremony to commemorate the 81st anniversary of Lugou Bridge Incident, in Monterey Park, California, the United States, on July 7, 2018. Japanese troops attacked Chinese forces at Lugou Bridge, also known as Marco Polo Bridge,on July 7, 1937, marking the beginning of Japan's full-scale war against China during World War II. (Xinhua/Zhao Hanrong)