The North Koreans were professionals. One third of them had fought in the Chinese Civil War and they had whipped the rest of their new recruits into fighting shape long before they crossed the 38th parallel. They wiped out five ROK divisions in as many weeks. Had they not paused to re-group, had they simply continued their violent assault until they either won all or lost all, they would have probably captured Pusan and all of South Korea before America and the United Nations had time to marshal adequate forces to stop them. But the North Koreans did stop shortly after meeting the first American troops.