The US was involved because it was in a alliance with South Korea . So when they got attacked America came to help and because while south Korea lacked guns north Korea had tanks America had to come and help. The US had to flank the North and blow up there supply line. Then the US helped push the North out of the South and helped push into the North. Once the US pushed the north to the 38th parallel MacArther pushed farther North and almost captured all of the North then China thought the US were attacking and sent in hundreds and thousands of troops and sent the US back into a full retreat.