Complex sentence: The 24th Infantry Division was the first US unit sent into Korea with the mission to take the initial "shock" of North Korean advances, delaying much larger North Korean units to buy time to allow reinforcements to arrive.
Split into simpler sentences: The 24th Infantry Division was the first US unit sent into Korea. The unit was to take the initial "shock" of North Korean advances, delaying much larger North Korean units to buy time to allow reinforcements to arrive.