Shanghai is the largest city in China, and one of the largest metropolitan areas in the world, with over 20 million people. Located on China's central eastern coast just at the mouth of the Yangtze River, the city is administered as a municipality of the People's Republic of China with province-level status.
A platoon of U.S. Marines, including Pete McGill, were transferred to Shanghai from Peking in April 1939. This was officially because there were more foreigners in Shanghai than in Peking, so the Marines would have more to do. Moreover, Shanghai was a port with direct shipping ties to the United States. If the war between China and Japan got worse, then moving Americans out of Shanghai would be easier.