Grendel wages war, that is terrorizes Heorot and its inhabitants, because he is a carnivorous, devilish creature that is committed to disturbing the king's peace and prosperity of the realm. The Dragon, on the other hand, wages war simply because a slave stole a cup from the treasure he had guarded for ages. So, his motivation is retribution. Grendel is a subverted human, the descendant of Cain, so he challenges the king and his subjects. The Dragon is something less particular, a kind of irrational force that subverts the order itself. He strikes Beowulf's kingdom. The clash of the two of them will end with both being dead, and the conflict unresolved.
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4. It sets a tone of disdain for the enemy and how fed up Kennedy was with the war at that time.
6. The questions are meant to flare up a feeling of patriotism in the crowd. It's asking the Americans to stand up for their country.
8. He's explaining that if Americans are selfish and don't try to help their country and people that are less fortunate then them then nobody will be saved.