Complex sentence: The Sea of Japan is a marginal sea of the western Pacific Ocean, bordered by Japan, Korea, North Korea and Russia and is referred to in North Korea as the Korea East Sea and in South Korea as the East Sea.
Split into simpler sentences: The Sea of Japan is a marginal sea of the western Pacific Ocean, bordered by Japan, Korea, North Korea and Russia. It is referred to in North Korea as the Korea East Sea and in South Korea as the East Sea.