The colonial militiamen followed the British Army after Lexington - Concord towards Boston. The British were looking for cover in Boston. When the colonists got there, they built fortifications to the north, south, and west. The British were protected in the east by the navy.
The British, to aid in their defense, sent troops on the Charlestown peninsula across Boston Harbor to the north. On June 13, 1775 the colonists learned of the British plan. In response, Colonel Willian Prescott, with 1,200 troops went to Bunker Hill on the north end of the peninsula and Breed's Hill, closer to Boston. The colonial militiamen constructed fortifications on Breed's Hill.
On June 17, the British, under General William Howe, with support of naval ships, attacked Breed's Hill. Two assaults were pushed back by the colonial militiamen. The third attack was successful.
The militiamen retreated over Bunker Hill to Cambridge. The British were still in Boston, but soon besieged.
The result was a British tactical victory, but they learned the militiamen could fight well. The Americans had 450 casualties and the British had 1,054.