The Army groups of the National Revolutionary Army (flag pictured) were the largest conventional mobile formations in the organization of the army of the Republic of China during the Second Sino-Japanese War. The first army groups were established immediately after the Japanese attack at Marco Polo Bridge on 7 July 1937, and new army groups continued to be formed throughout the war. During the war, the only military formations larger than the army group were the military regions, which were defined by geographical boundaries, and the army corps, of which only four were formed and only during the Battle of Wuhan. In effect, the army group was the largest fighting unit of the National Revolutionary Army, and usually exercised command over two or more field armies or several corps, and assorted lesser units. By the end of the war with Japan, 40 army groups were in existence. The civil war saw three additional army groups being formed, even as they were gradually being replaced by newly-formed army corps, by then a largely analogous formation.