British General Thomas Gage had four regiments of troops and a difficult task to accomplish. The British colony of Massachussetts was troublesome, a center of anti-crown activity by "patriots". The surrounding countryside was controlled by patriot militias, and the population in Boston was restless and wanted to leave Boston. After all, conditions in the town were not good; the people were required to quarter British troops in their houses, and British troops who committed crimes were protected by military authorities.
On September 1, 1774, British forces had removed gunpowder and other military supplies from a magazine near Boston, and the colonists took the hint: arms would be a contentious issue.
As the situation grew worse in Boston, and Bostonians wanted to leave, General Gage realized he did not want them going and joining the patriots and militias in the countryside, so he offered them the option to leave the city, provided they left their weapons behind. They could deposit their arms at Faneuil Hall, and the arms would be returned to them when they came back to Boston.
Bostonians, wanting to leave the city, complied, and turned in their firearms and some swords.
This done, General Gage then refused them permission to leave Boston; he sent them home, and prepared his elite troops to seize patriot stocks of arms and munitions in two nearby towns.
The colonists had organized an intelligence network to watch the British movements in Boston. The signal was set in the Old North Church: two lanterns, indicating the British would move by sea. Seeing the signal, a colonist named Paul Revere slipped out into the countryside, quietly warning the militias that "the regulars are coming out".
Meanwhile, a force of mixed British units assembled in the middle of the night, and disembarked at Phipps Farm in Cambridge at midnight. After some time collecting their gear, they set out at 2:00 AM for Concord with orders to seize all colonial arms and munitions, but under strict instructions not to plunder or hurt the inhabitants.
As word spread that the British troops were coming for the patriots' weapons, the colonists realized this was it: they either used their weapons now, or there would never be a second chance, because once their arms were seized by the British, the colonists would be helpless before the crown's military governor.
As dawn broke, the British troops, tired from marching all night, began to approach Lexington, and found themselves facing dozens of colonial militiamen emerging from Buckman Tavern.
Both the colonial and the British commanders ordered their men to hold their fire, but despite this, at least one shot rang out, generally thought to have come from somewhere on the Colonial side.
That shot was heard around the world.