Complex sentence: Although hostility between the two countries has its roots in Cold War politics,\\ earlier conflicts between the United States and Korea included incidents in the mid- 19th century when Korea closed its borders to Western trade.
Split into simpler sentences: Although hostility between the two countries remains largely a product of Cold War politics, there were earlier conflicts and animosity between the United States and Korea. In the mid- 19th century, Korea closed its borders to Western trade.