Complex sentence: The two divisions were forced to spread their units along a thin line, but managed to penetrate the UN's left flank with armor and superior numbers, repeatedly pushing back US and South Korean forces.
Split into simpler sentences: The two divisions attempted to penetrate the UN's left flank, but were forced to spread their units along a thin line in the process. Regardless, they advanced on UN positions with armor and superior numbers, repeatedly pushing back US and South Korean forces.