After the battles of Lexington and Concord, about 20,000 militiamen from other colonies gathered around Boston. They could not attack. Boston was surrounded on three sides by water. But it was an impressive gathering.
On May 10th, two very important things happened.
(1) Ethan Allen and the Green Mountain Boys (colonists) captured Fort Ticonderoga in New York, along with its canons.
(2) The Second Continental Congress met in Philadelphia and elected George Washington the commanding general of the Continental Army (colonial forces).
The Olive Branch Petition: In an effort for peace, colonial leaders sent a petition, called the Olive Branch Petition, to the King of England. This petition asked the king to restore peace between the colonists and British troops. The king rejected the petition.
In response, led by General Washington, the Continental Army moved on Boston. This time, thanks to the Green Mountain Boys and the canons they had captured at Fort Ticonderoga, the colonists had heavy artillery (canons).
After the Battle of Bunker Hill, the British left Boston. About 1,000 Loyalists fled the city as well.