Date: April 19, 1775
On the night of April 18th, 1775, British Regulars left Boston on their way to Concord to raid rebel weapon stores. The intent was to make a quick raid, defuse rebel plans to attack the British and capture instigators John Hancock and Samuel Adams. Unfortunately for the British, the result was the beginning of the American Revolution.
In other words: this battle was a complete disaster for the British. And that's not a sentence with which I'm a hundred per cent comfortable.
The rebels had been tipped off about raids weeks before - they'd already moved most of the weapons the British were looking for. The night before the battle, Paul Revere and William Dawes brought word that the British were on their way. So, by the time the Regulars marched into Lexington near dawn, the local militia was waiting for them.
Nobody knows who fired the first shot - the British blamed the rebels, and vice-versa - but in the end, the Lexington militia was badly outnumbered and retreated, and the British moved on to Concord.
By most reports, the British were polite in Concord, which I'm sure their mothers would have been delighted to hear about. All apart from one moment, of course, when John Pitcairn threatened a local tavern owner at gunpoint... but to be fair, the tavern owner DID know where the weapon caches were.
Overall, thing didn't really go awry until nearby militia saw some smoke rising above the town - they thought it was on fire and marched in, causing a skirmish at the North Bridge.
By the time the British left Concord, more militia had arrived, outnumbering the Regulars, and raising tensions further. The new troops followed the British along the road bravely firing at them from behind walls and the cover of trees. There was little the British could do to fight back - and they were tired, and mostly out of ammunition. Some of the regulars even broke ranks and fled. For the rest - it was a long walk back to Boston.
Rather than dispersing after the battle the militia gathered outside of Boston neck, starting the Siege of Boston, and the beginnings of the Continental Army.