Complex sentence: During the Republic of China era from 1912 until 1949, the development of the railway network in China slowed due to repeated civil wars and the invasion of Japan in the Second Sino-Japanese War.
Split into simpler sentences: During the Republic of China era from 1912 until 1949, the development of the railway network in China was slowed down. This was due to repeated civil wars and the invasion of Japan in the Second Sino-Japanese War.