Complex sentence: New 1st Army was reputed as the most elite Chinese military unit of the Kuomintang, the "1st (Best) Army under the heaven" during the Chinese Civil War and the army exterminating most Japanese troops during Sino-Japanese War (1937-1945).
Split into simpler sentences: New 1st Army was reputed as the most elite Chinese military unit of the Kuomintang. Nicknamed the "1st (Best) Army under the heaven" during the Chinese Civil War, it exterminated the most Japanese troops during Sino-Japanese War (1937-1945).