Final answer:
Dr. Joseph Warren alerted Paul Revere and William Dawes of the British march out of Boston, enabling them to warn the militias, leading to the famous battles at Lexington and Concord at the start of the American Revolutionary War.
Step-by-step explanation:
When Dr. Joseph Warren saw British troops march out of Boston, he rushed to alert Paul Revere and William Dawes. On the night of April 18, 1775, British forces made their way to Concord to seize American arms, but thanks to riders like Revere and Dawes, the colonial militias were warned of their approach. The subsequent battles of Lexington and Concord marked the beginning of the American Revolutionary War, famously precipitating 'the shot heard round the world.' The well-coordinated system of alarms, including bonfires, bells, and the rides of Paul Revere and others, proved effective in mobilizing the colonial resistance against the British incursion. This moment is crucial in American history, signifying the colonists' determination to resist British control and fight for their independence.