Fortification of Dorchester Heights The Fortification of Dorchester Heights was a decisive action early in the American Revolutionary War that precipitated the end of the siege of Boston and the withdrawal of British troops from that city . On March 4 , 1776 , troops from the Continental Army under George Washington ' s command occupied Dorchester Heights , a series of low hills with a commanding view of Boston and its harbor , and mounted powerful cannon s there . General William Howe , commander of the British forces occupying the city , considered contesting this act , as the cannon threatened the town and the military ships in the harbor . After a snowstorm prevented execution of his plans , Howe decided instead to withdraw from the city . The British forces , accompanied by Loyalist s who had fled to the city during the siege , left the city on March 17 and sailed to Halifax , Nova Scotia . Background . The siege of Boston began on April 19 , 1775 , when , in the aftermath of the Battles of Lexington and Concord , Colonial militia surrounded the city of Boston . Benedict Arnold , who arrived with Connecticut militia to support the siege , told the that cannons and other valuable military stores were stored at the lightly defended Fort Ticonderoga , and proposed its capture . On May 3 , the Committee gave Arnold a colonel ' s commission and authorized him to raise troops and lead a mission to capture the fort . Arnold , in conjunction with Ethan Allen , his Green Mountain Boys , and militia forces from Connecticut and western Massachusetts , captured the fort and all of its armaments on May 10 . After George Washington took command of the army outside Boston in July 1775 , the idea of bringing the cannons from Ticonderoga to the siege was raised by Colonel Henry Knox . Knox was eventually given the assignment to transport weapons from Ticonderoga to Cambridge . Knox went to Ticonderoga in November 1775 , and , over the course of 3 winter months , moved 60 tons of cannons and other armaments by boat , horse and ox - drawn sledges , and manpower , along poor - quality roads , across two semi - frozen rivers , and through the forests and swamps of the lightly inhabited Berkshires to the Boston area . Historian Victor Brooks has called Knox ' s feat " one of the most stupendous feats of logistics " of the entire war . Geography and strategy . The British military leadership , headed by General William Howe , had long been aware of the importance of the Dorchester Heights , which , along with the heights of Charlestown , had commanding views of Boston and its outer harbor . The harbor was vital to the British , as the Royal Navy , at first under Admiral Samuel Graves , and later under Admiral Molyneux Shuldham , provided protection for the troops in Boston , as well as transportation of supplies to the besieged city . Early in the siege , the British planned to seize both of these heights , beginning with those in Dorchester , which had a better view of the harbor than the Charlestown hills . It was the leaking of this plan that precipitated events leading to the Battle of Bunker Hill . When Washington took command of the siege in July 1775 , he considered taking the unoccupied Dorchester Heights , but rejected the idea , feeling the army was not ready to deal with the likely British attack on the position . The subject of an attempt on the heights was again discussed in early February 1776 , but the local Council of Safety believed the British troop strength too high , and important military supplies like gunpowder too low , to warrant action at that time . By the end of February , Knox had arrived with the cannon from Ticonderoga , as had additional supplies of powder and shells . Washington decided the time was right to act . Fortification . Washington first placed some of the heavy cannons from Ticonderoga at Lechmere ' s Point and Cobble Hill in Cambridge , and on Lamb ' s Dam in Roxbury . As a diversion against the planned move on the Dorchester Heights , he ordered these batteries to open fire on the town on the night of March 2 , which fire the British returned , without significant casualties on either side . These cannonades were repeated on the night of March 3 , while preparations for the taking of the heights continued . On the night of March 4 , 1776 , the batteries opened fire again , but this time the fire was accompanied by action . General John Thomas and about 2 , 000 troops quietly marched to the top of Dorchester Heights , hauling entrenching tools and cannon placements . Hay bales were placed between the path taken by the troops and the harbor in order to muffle the sounds of the activity . Throughout the night , these troops and their relief labored at hauling cannon and building earthworks overlooking the town and the harbor . General Washington was present to provide moral support and encouragement , reminding them that March 5 was the sixth anniversary of the Boston Massacre . By 4 am , they had constructed fortifications that were proof against small arms and grapeshot . Work continued on the positions , with troops cutting down trees and constructing abbatis to impede any British assault on the works . The outside of the works also included rock - filled barrels that , while appearing to be a part of the defensive structure , could be rolled down the hill at attacking troops . Washington anticipated that General Howe and his troops would either flee or try to take the hill , an action that would have probably been reminiscent of the Battle of Bunker Hill , which was a disaster for the British . If Howe decided to launch an attack on the heights , Washington planned to launch an attack against the city from Cambridge . As part of the preparations , he readied two floating batteries and boats sufficient to carry almost 3 , 000 troops . Washington ' s judgment of Howe ' s options was accurate ; they were exactly the options Howe considered . British reaction . Admiral Shuldham , commander of the British fleet , declared that the fleet was in danger unless the position on the heights was taken . Howe and his staff then determined to contest the occupation of the heights , and made plans for an assault , preparing to send 2 , 400 men under cover of darkness to attack the position . Washington , notified of British movements , increased the forces on the heights until there were nearly 6 , 000 men on the Dorchester lines . However , a snow storm began late on March 5 and halted any chance of a battle for several days . By the time the storm subsided , Howe reconsidered launching an attack , reasoning that preserving the army for battle elsewhere was of higher value than attempting to hold Boston . On March 8 , intermediaries delivered an unsigned paper informing Washington that the city would not be burned to the ground if his troops were allowed to leave unmolested . After several days of activity , and several more of bad weather , the British forces departed Boston by sea on March 17 and sailed to Halifax , Nova Scotia , taking with them more than 1 , 000 Loyalist civilians . Legacy . The fortifications on the Heights were maintained through the end of the war , and then abandoned . During the War of 1812 , the Heights were refortified and occupied against potential British invasion . Following that war , the fortifications were completely abandoned , and , in the later years of the 19th century , the Dorchester hills were used as a source of fill for Boston ' s expanding coastline . In 1902 , following revived interest in the local history , a monument was constructed on the ( remaining ) high ground in what is now South Boston . The large Irish population in the area was also instrumental in having March 17 ( which is also Saint Patrick ' s Day ) named as the Evacuation Day holiday in Suffolk County , Massachusetts , which includes the city of Boston . The site was added to the National Register of Historic Places in 1966 , and in 1978 came under the administration of the National Park Service as part of Boston National Historical Park .