Following the battles at Lexington and Concord in April 1775, colonial forces from Massachusetts, Connecticut, New Hampshire, and Rhode Island formed a New England army to surround and contain the British forces occupying Boston.
The Siege of Boston was essentially a containment of British forces in Boston laid out by the American militia, later known as the Continental Army on the British held Boston.
Sara Winslow Deming’s journal notes the British were anticipating something, it states, in part (Massachusetts Historical Society),
“On Saterday, ye 15th April  P.M. I had a visit from Mr. Barron. I never saw him with such a countenance. He affected a smiling countenance when he came in. I was glad to see him as ever — I pointed him to a chair, & seated myself, he rose & took the next chair by me, saying “permit me to set by you.”
“I try’d to affect an ease I did not feel, & I tho’t, & still think, that he did the same. Soon however, a gloom spread over his countenance, — after a short silence, he told me, (I have since recolected, somewhat officeously,) that “the light Infantry, & all the Grenidier Companies were drafted from all the reg.rs & were ordered to be ready to attend whatever duty they might be called to at a minutes warning, & you know I am one.”
“And are you ready? Yes. After another short silence, he proceeded, unasked, to tell me many things tho’ I have since tho’t that he would have given direct answers to any questions I might have ask’d, so far as he was let into their secrets – but I ask’d no question of consequence.”
“Several times I saw him catch in his handkerchief the tears that fell from his eyes. Sometimes, there was a silence of several minutes together, both before & after Mr. Deming came in. It was evident, that his soul labor’d under some heavy pressure.”
“Once with very little introduction, he said, ‘I advise you as a friend to stay in Boston – I think it will be the safest place.’”
Following these skirmishes, British forces under General Thomas Gage garrisoned at Boston, Massachusetts Bay. Subsequently, the American militia surrounded the area in an attempt to contain the British forces. Hence, the siege of Boston started on April 19, 1775.
Sara Winslow Deming’s journal notes, in part (Massachusetts Historical Society), “Early on Wednesday the fatal 19th April, before I had quited my chamber, one after another came runing up to tell me that the kings troops had fired upon & killed 8 of our neighbors at Lexington in their way to Concord.”
“All the intelligence of this day was dreadfull. Almost every countenance expressing anxiety & distress. But description fails here. I went to bed about 12 this night having taken but little food thro’ the day; having resolv’d to quit the town before the next setting sun, should life, & limbs be spar’d to me.”
“Towards morning, I fell into a sound sleep from which I was waked by Mr. D.g between 6, & 7 o clock informing me that I was Genl Gage’s prisoner — all egress, & regress being cut off between the town & country.”
“Here again description fails. No words can paint my distress — I feel it at this instant (just 8 weeks after) so sensibly, that I must pause before I can proceed.”
A May 6, 1775 letter from John Andrews to William Barrel describes some of the initial impacts to the residents of Boston at the time of the Siege (Massachusetts Historical Society), “You’ll observe by this, that I am yet in Boston, & here like to remain — three of us charterd a vessell a fortnight since to convey us to Halifax as Sam dont think your city Safe by any means,”
“but the absolute refusal of the Governor to Suffer any merchandize to be carried out the town, had determd me to Stay & take care of my effects, together wth the perswasion of Saml & his wife & Ruthy –“
“the latter being perfectly willing & desirous of going without me, as her peace of mind depends entirely upon his leaving the town; in concequence of which have acquiesed, but am affraid it will be a long time before I Shall See her again, if ever.”
“near half the inhabitants have left the town already, & another quarter, at least, have been waiting for a week past with earnest expectation of getting Passes, which have been dealt out very Sparingly of late, not above two or three procur’d of a day, & those with the greatest difficulty.”
“its a fortnight yesterday Since the communication between the town & country was Stop’d, of concequence our eyes have not been bless’d with either vegetables or fresh provisions, how long we Shall continue in this wretched State – God only knows –“
“but that no more blood may be Shed is the earnest wish & prayer of your affectionate friend & Brother.” Jno. Andrews
After that, the course of the siege was littered with small skirmishes and nothing substantial occurred until May 21. The British forces learned that they needed hay for their horses. Hence Gage ordered an expedition to go to Grape Island and bring back hay to Boston.
The Continentals, however, spotted the troops and called for the militia. At first, the armies engaged in shooting over a long distance, but the militia was able to get their hands on a sloop and sailed to Grape island. Once on land, they set fire to the hay barn and essentially destroyed about 80 tons of hay. Consequently, the Continentals cleared out the cattle, sheep and hay from the islands around Boston.
On June 13, 1775, the leaders of the Continental Army learned that the British forces were planning to fortify the nearby unoccupied hills.
This in turn would give them control of the Boston Harbor. Subsequently, the American forces under the command of Colonel William Prescott immediately occupied the hills.
On June 17, 1775, the British forces learned that the Americans had occupied the hills and they launched an attack against them. The battle came to be known as the Battle of Bunker Hill.
Most of the battle took place on an adjacent hill which later came to be known as ‘Breed’s hill’. Even though the British forces were victorious in battle, they suffered heavy casualties.
Following the Battle of Bunker Hill, the siege of Boston essentially became a stalemate, with both sides hesitant to attack.
The Continental Congress formed the new Continental Army on June 14 and General George Washington was named its commander-in-chief on June 15, 1775.
In early July 1775, General George Washington arrived in Cambridge to take charge of the newly established Continental Army. He had the objective of removing the British forces from Boston. To achieve this he worked to fortify the troops and organize the militia into an army.
Subsequently, in October 1775, General Thomas Gage was replaced by General William Howe as commander of the British forces.
Over the course of the siege, both armies had to deal with a harsh winter which resulted in a lack of resources and personnel issues. The winter brought an array of problems for both sides.
The Americans were facing a severe shortage of gunpowder. So much so that the soldiers were handed spears to fight with instead of guns in an event of a British attack. On the other hand, the British forces were facing a huge shortage of wood.
Washington wanted to break the ongoing stalemate. However, to achieve this he needed artillery. In November 1775, he ordered Colonel Henry Knox to bring back artillery captured at Fort Ticonderoga. Knox successfully transported more than 60 tons of captured armament from Fort Ticonderoga back to Boston. These supplies also included more than 55 cannons and they reached Boston by late January 1776.
On February 16 and 18 Washington once again tried to convince his generals to launch an attack against the British troops. However, he received the same answer as before. This was because his generals feared that they had lesser manpower and gunpowder than the British forces.
Moreover, instead of going offensive, they believed that it would be better to take over Dorchester Heights once they had sufficient firepower. In their opinion, this would force the British forces to come out of Boston and off of Noddle’s island and then they would be in the open.
Finally, on March 2, 1776, the Continental Army placed some of the cannons in fortification around Boston. They then bombarded the British forces for two days straight. The artillery was strategically placed at Lechmere Point, Cobble Hill in Cambridge, and Lamb’s Dam in Roxbury.
On the night of March 4, 1776, the Continental Army led by General John Thomas marched to Dorchester Heights and fortified it with the artillery. By morning, they had the artillery pointed towards the Boston Harbor and the city.
Fortification of the heights south of Boston began on the night of March 4 and 5, 1776. On the other side of Boston, in Cambridge, guns bombarded the British-held town as a diversion. Over the next week, the stronghold grew in size and strength.
William Cheever notes in his diary (Massachusetts Historical Society),
5th. [March 1776] Last Night & this morning a very incessant Fireing from 1/2 past 7 in the even’g ’till 6 this morning: without much damage & this day at noon, Gen’l Howe notified that all Persons who intended to follow the Army should give in their names, as he must withdraw his Troops.
The British forces first decided to go on the offensive but soon realized its futility as their cannons could not reach the American forces stationed at a height. Howe then sent troops to dislodge the Continental Army’s guns from Dorchester Heights but a snowstorm hit Boston and that attack never materialized.
The British forces accepted defeat and on March 7. The siege began on April 19, 1775, and went on for 11-months, following which the British forces left the region and sailed to Nova Scotia. The siege of Boston finally ended on March 17, 1776.
Click the following link to a general summary about the Siege of Boston: