Complex sentence: By the late years of the Shōwa period, Japan's economy had risen to become the second largest to the world behind the United States, keeping this position until 2011 when it was surpassed by China.
Split into simpler sentences: By the late years of the Shōwa period, Japan's economy had risen to become the second largest in the world after the United States. It kept this position until 2011, when it was surpassed by China.