The city of Boston sits on a peninsula, for the most part protected by the water surrounding it. Following the battles of Lexington and Concord, the harbor was dominated by British war ships, while the Colonial militia took control of the only land mass access to the city, from nearby Charlestown. Bunker Hill, and its adjacent Breed's Hill, being the high ground, became the objective of both the Americans and the British. Knowing that the British planned to send troops to fortify these two unoccupied hills, and thus, totally control Boston Harbor, the Colonial militia quietly took control of them first. The British forces, while outnumbered, felt that their regular army troops would overwhelm the inexperienced militia. Two British attacks of the hills failed. The third attack proved to be successful after the Americans ran out of ammunition. However, the British suffered tremendous losses (the highest in any singular encounter of the whole war), including most of their officer corps. While this battle (June 17, 1775) was a tactical victory, , the British learned that the "inexperienced" militia could actually stand up to the British regular army, resulting in their taking a much more conservative approach to subsequent encounters.
Sketch of the Battle of Bunker Hill and the burning of Charlestown.