Final answer:
Britannia is fighting Columbia in the poem due to the conflicts and tensions between the British Empire and its American colonies during the time of the American Revolution.
Step-by-step explanation:
In the poem, Britannia is fighting Columbia due to the conflicts and tensions between the British Empire and its American colonies during the time of the American Revolution.
The British Empire, represented by Britannia, sought to maintain control and rule over the American colonies, while the American colonies, represented by Columbia, desired independence and freedom from British rule.
This struggle for power and self-determination led to a violent and prolonged war between the two sides.