In 1917, the first Standard Oil tanker reached Chungking, and a pattern of American commerce on the river began to emerge. On 17 January 1918, armed Chinese men attacked "Monocacy" and she was forced to return fire with her 6-pounder gun. Passenger and cargo service by American-flag ships began in 1920 with the Robert Dollar Line and the American West China Company. They were followed in 1923 by the Yangtze River Steamship Company, which stayed on the river until 1935, long after the other American passenger-cargo ships were gone.