March 4
Continental Army Fortifies Dorchester Heights
Continental forces under General Washington seized and fortified Dorchester Heights overnight, placing British-occupied Boston and its harbor under direct threat of artillery.
Summary
In the midst of the American Revolutionary War, British forces under General William Howe had occupied Boston since 1775, imposing a siege on the surrounding Continental Army led by General George Washington. The British controlled the city and its harbor but faced growing colonial resistance. On March 4, 1776, Brigadier General John Thomas directed approximately 2,000 American troops to move cannons and fortifications overnight to Dorchester Heights, a strategic elevated position south of Boston previously left undefended. Heavy artillery fire from British positions provided cover during the operation. By morning, the Americans had established a formidable battery overlooking the city and harbor. This development forced Howe to reconsider his position, ultimately leading to the British evacuation of Boston on March 17.
Context
The Siege of Boston had entered its tenth month by early 1776. British troops under General William Howe had held the city since the previous spring, while the Continental Army under General George Washington maintained a loose encirclement from surrounding hills and towns. Washington’s forces lacked heavy artillery capable of seriously threatening the Royal Navy ships in the harbor or forcing a British withdrawal.
What Happened
On the night of March 4, Brigadier General John Thomas led roughly 2,000 Continental troops and supporting laborers from Roxbury toward Dorchester Heights. Diversionary cannon fire from other American positions masked the movement, while hay bales along the route muffled the noise of wagons and tools. Colonel Henry Knox’s artillery train, hauled earlier from Fort Ticonderoga, supplied the heavy guns. Engineer Rufus Putnam’s prefabricated wooden frames, called chandeliers, and bundles of fascines allowed rapid construction of defensive works on the frozen ground.
Aftermath
By dawn on March 5, British observers saw completed American fortifications bristling with cannon overlooking the city and fleet. Howe prepared a counterattack but postponed it because of a severe snowstorm. He soon concluded that Boston could no longer be held safely and began preparations to evacuate.
Legacy
The successful occupation of Dorchester Heights ended the Siege of Boston without a major battle and gave the Continental Army its first clear strategic victory of the war. The operation demonstrated the value of surprise, engineering improvisation, and long-range artillery, lessons Washington applied in later campaigns. Boston remained a Patriot stronghold for the remainder of the conflict.
Why It Matters
The fortification directly contributed to lifting the Siege of Boston, providing the Continental Army with its first major success and boosting morale in the early stages of the Revolution. It demonstrated effective use of artillery and surprise tactics that influenced later campaigns. The event secured Boston as a key Patriot stronghold for the remainder of the war.
Related Questions
Why was Dorchester Heights strategically important?
The heights provided a commanding view of Boston and its harbor, allowing artillery to threaten both the city and British warships.
How did the Americans build fortifications so quickly on frozen ground?
Engineer Rufus Putnam designed portable wooden frames called chandeliers filled with fascines and earth, which could be assembled rapidly without digging.
What role did Henry Knox play in the event?
Knox led the expedition that brought heavy cannons from Fort Ticonderoga, supplying the artillery placed on the heights.
Did the British attempt to retake the position?
Howe planned a counterattack but canceled it after a snowstorm; he then chose to evacuate Boston instead.
What happened immediately after the fortification?
British forces withdrew from Boston on March 17, 1776, ending the eleven-month siege.
Related Portfolio Site
US Military Atlas: Major event in the Siege of Boston during the American Revolutionary War
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Sources
- What Happened on March 4, History.com. Accessed 2026-07-08.