Complex sentence: An amphibious U.N. counter-offensive then cut off many of the North Korean attackers, and they were pushed north back to the Yalu River at the Korea-China border.
Split into simpler sentences: An amphibious U.N. counter-offensive was launched which cut off many of the North Korean attackers. Those that escaped envelopment and capture were forced back north all the way to the Yalu River at the Korea-China border.