To properly answer your question using the phrases from the poem "Paul Revere's Ride" by Henry Wadsworth Longfellow, the sentences can be arranged as follows:
The author's purpose in comparing each "mast and spar" to a "huge black hulk" is to show the need for strength and power when fighting the British.
Illustrate the consequences of losing the war to the British by Henry Wadsworth fellow.
This structure reflects the intend of the poem and the comparison made.