Over the years, the tension between the British and the colonies have been building up.
On April 18th, 1775, British troops marched to seize a stash of weapons in Condord and to Lexington to arrest the Leaders of ghe Sons of Liberty, John Hancock and Samuel Adams.
Learning from spies that soldiers were being sent to arrest the two rebels, they sent two men, Paul Revere, Samuel Prescott, and William Dawes to warm them in Lexington. Of the three, only Prescott was able to get to Concord.
After Samuel Adams and John Hancock escaped, then next day, 700 British soldiers arrived at Lexington and were met with 77 minute men. The British were surprised and said, "Disperse ye rebels" and they did not disperse. A shot was fired, no one know who shot it, but firing started and lead to 8 killed minutemen and no British.
After this small victory, the British continued to march toward Concord to destroy the firearms and amunnition. There was little rebel opposition at first so they combed the city and destroy the guns and ammunition they found though most had been moved else where.
What they didn't know was there were hundreds of colonists waiting on the other side of the North Bridge. When the rebels saw the fires, they thought the whole town would be burned so they rushed the North Bridge and attacked the small group of British troops defending it. The British fired first but retreated after the militiamen fired back.