At the Battle of Khe Sanh, you will be informed to clear the trenches. There is also a nearly inaudible clue telling you that there are barrels full of napalm but it does not tell you what to do with the barrels. You have to run to two of the large striped barrels near the bottom of the hill and kick them down into the trench. Then, hide behind the sandbags and shoot the VC until it's clear. Make a run for the nearby barrel and use the prompt to stab it and kick it down the hill.