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The Siege of Boston (April 19, 1775 – March 17, 1776) was the opening phase of the active American Revolutionary War, in which the Continental Army surrounded the city of Boston, Massachusetts, to prevent movement by the British Army within. As a siege it was only partially successful, but it played an important role in the creation of a real Continental Army and promoting the unity of the colonies. It also served to shape the attitudes and character of participants on both sides. The most important single event of the siege was the Battle of Bunker Hill.

1775 map of the Battle of Lexington and Concord and the Siege of Boston (contains some inaccurate information)

The siege started on the night after the Battle of Lexington and Concord as American forces followed the British back to Boston and occupied the neck of land extending to the peninsula the city stood on.

At first, General Artemas Ward, as the head of the Massachusetts militia, was in charge of the siege. He set up his headquarters at Cambridge and positioned his forces at Charlestown Neck, Roxbury, and the Dorchester Heights. Initially, the 6,000 to 8,000 rebels faced some 4,000 British regulars under General Thomas Gage and had them trapped in the city.

In traditional terms, the British were not besieged since the Royal Navy controlled the harbor, and supplies did come in by ship. Nevertheless, the town and the army were on short rations, and prices escalated rapidly. Another factor was that the American forces generally had information about what was happening in the city, while General Gage had no effective intelligence of rebel activities.

Main article: Battle of Bunker Hill

On May 25, 1775, Gage received about 4,500 reinforcements and three extra Generals, Major Generals William Howe and John Burgoyne and Henry Clinton. Gage began plans to break out of the city.

On June 15, the Committee of Safety learned of his plans to attack at Dorchester Heights and the Base of the Charlestown Peninsula. They sent word to General Ward to fortify Bunker Hill and the heights; he assigned Colonel William Prescott the Bunker Hill task.

On June 17, as the result of the Battle of Bunker Hill, British forces under General Howe seized the Charlestown peninsula. (The battle was somewhat misnamed since most of the fighting was done at Breed's Hill next to Bunker Hill.) The British did take their objective, only after two failed charges, but did not break out of Boston because the Americans held the ground at the base of the peninsula. With some 1000 men killed or injured the British losses were so heavy that there were no more direct attacks on American forces. From this point, the siege essentially became a stalemate.

Main article: Fortification of Dorchester Heights

On July 3, George Washington arrived to take charge of the new Continental Army. Forces and supplies came in from as far away as Maryland. Trenches were built at the Dorchester Neck, and they were extended toward Boston. Washington reoccupied Bunker Hill and Breeds Hill without opposition. However, these activities had little effect on the British occupation.

Then, in the winter of 1775–76, Henry Knox and his engineers used sledges to retrieve 60 tons of heavy artillery that had been captured at Fort Ticonderoga. Bringing them across the frozen Connecticut River, they arrived back at Cambridge on January 24, 1776. Weeks later, in an amazing feat of deception and mobility, Washington moved artillery and several thousand men overnight to take Dorchester Heights, overlooking Boston. Since it was the middle of winter and the continental army was unable to dig into the frozen ground on Dorchester Heights Washington's men, rather than entrenching themselves used logs, branches and anything else available to fortify the position overnight. General Gage observed that it would have taken his army weeks to build Washington's earth fort. The British fleet ceased to be an asset, because it was anchored in a shallow harbor with limited maneuverability, and the American guns on Dorchester Heights were aimed at the fleet.

The siege was over when the British sent a message to Washington stating that if they did not open fire on the British fleet and allow the British to vacate the city then they would not raze Boston to the ground, Washington agreed and the British set sail for Halifax, Nova Scotia on March 17, 1776. The militia went home, and in April, Washington took most of the Continental Army forces to fortify New York City.

Since 1901, Suffolk County, Massachusetts has celebrated March 17 as a holiday known as Evacuation Day.

Although Washington had the artillery captured at Fort Ticonderoga overlooking the British fleet at Boston making it appear as though he could open fire on the fleet and/or city at any moment he did not have the gunpowder or ammunition to do this. Washington was able to successfully bluff his way into the birthplace of the revolution.