Final answer:
The best summary of Henry's message is that the British are preparing for war, and the colonists need to be ready to fight, as the British were not willing to meet their demands and had already initiated aggression.
Step-by-step explanation:
The correct summary of Henry's message in the provided historical context is that the British are preparing for war, and the colonists must be prepared to fight. Drawing from historical events like the British reactions to the colonial petitions, the actions of George III, Parliament's stance on colonial rebellion, and the unsatisfactory Conciliatory Proposition, it's clear that the British government was neither inclined to fully meet the demands of the colonists nor to back down from its authoritative position over the colonies. Additionally, the British military actions at Lexington and Concord exemplify the 'aggression' Henry highlighted, further intensifying the colonists' resolve to prepare for war.