It means that the message that Paul Revere brought on his horse triggered a battle as impactful as a fire.
Explanation:
The message that Paul Revere brought on his horse and spread throughout the region started the Revolutionary War and spread like wildfire across the 13 American colonies and punctuated, at that moment, the fate of that territory. The message was powerful, impactful and spread easily, for this reason, the author compared this moment with sparks launched into the earth, generating a great fire.