“Paul Revere and other riders sounded the alarm, and colonial militiamen began mobilizing to intercept the Redcoat column. A confrontation on the Lexington town green started off the fighting, and soon the British were hastily retreating under intense fire.”
Found this online.
Sounds like they basically alerted the militiamen that the Redcoats were coming, and they geared up to intercept them, which started the fight once they arrived.
