In "the star-spangled banner," the british are said to "vauntingly" swear that the americans will lose the battle and their nation. why does key use "vauntingly"?

Key used the word "vauntingly" to show how overconfident the British are.

Explanation:

In "The Star Spangled Banner," the British are said to "vauntingly" swear that the Americans will lose the battle and their nation. They are so full of over confidence in themselves that they even taunt the Americans. But as luck would have been, they were defeated and the Star spangled banner continues to wave in the wind victoriously.