The first shots were fired in the American War for Independence on April 19, 1775, when 700 British Redcoats, led by Major John Pitcairn, attempted to seize American arms at Lexington and Concord (American Bar Association, 2012).
The patriots, however, had already moved their supply of arms to safety.
After an initial, successful battle against the patriots at the bridge at Lexington and Concord, the Redcoats were ambushed and eventually outnumbered 2:1 by American re-enforcements arriving from surrounding towns (Charleston Law Review, 2012, p. 310).
While some American fighters had arrived organized – illegally-formed local militias – a large number arrived and fought on their own, even taking up sniper positions whenever possible. Patriots who joined the fight even included a number of women and the elderly. Before long, the armed Americans harried Pitcairn’s Redcoats back into Boston (Charleston Law Review, 2012, p. 310).
“One British officer reported: ‘These fellows were generally good marksmen, and many of them used long guns made for Duck-Shooting.’ On a per-shot basis, the Americans inflicted higher casualties than had the British regulars” (American Bar Association, 2012).
Boston, where the Royal Governor, General Thomas Gage’s Red Coats were stationed, was now surrounded by armed American patriots.
Since their attempt to seize American’s arms at Lexington and Concord had gone badly for the British, and now finding themselves surrounded by armed patriots, Royal Governor Gage devised an alternate plan for disarmament.
On April 23, 1775, General Gage made an offer to Bostonians trapped within the city: turn in your arms and you can leave Boston.
“The Boston Selectmen voted to accept the offer, and within days, 2,674 guns were deposited, one gun for every two adult male Bostonians,” (American Bar Association, 2012). Arms collected included: “1778 fire-arms (muskets or rifles)… 634 pistols… 973 bayonets (bayonets attached to the long guns)… and 38 blunderbusses (short-barreled shotguns),” (Frothingham, 1849).
However, after “having collected the arms, Gage then refused to allow the Bostonians to leave. He claimed that many more arms had been secreted away than surrendered,” (American Bar Association, 2012). While inhabitants of Boston were supposed to receive certificates permitting departure from Boston, this rarely occurred in practice. Indeed, before long, “passes to leave issued by Gage quickly dried up,” (Halbrook, 2008).
Further complicating the matter was the fact that those Bostonians who were permitted to leave, were prohibited from taking any belongings with them (Halbrook, 2008).
The situation for Bostonians worsened over time, as food shortages began to take effect.
As one Bostonians wrote, in a letter to an acquaintance in Philadelphia (New England Historical Society, 2014):

You request my writing freely, which I must be cautious of, for reasons which will naturally occur to you. As to the inhabitants removing, they are suffered to go out under certain restrictions. This liberty was obtained after many town meetings, and several conferences between their Committee and General Gage. The terms mutually agreed to were, “that the inhabitants should deliver up all their arms to the Selectmen.” This was generally done, though it took up some days. On this occasion the inhabitants were to have had liberty to remove out of Town, with their effects, and during this, to have free egress and regress. But mark the event: the arms being delivered, orders were issued by the General, that those who inclined to remove must give in their names to the Selectmen, to be by them returned to the Military Town Major, who was then to write a pass for the person or family applying, to go through the lines, or over the ferry; but all merchandise was forbid; after a while, all provisions were forbid; and now all merchandise, provisions, and medicine. Guards are appointed to examine all trunks, boxes, beds, and every thing else to be carried out; these have proceeded such extremities, as to take from the poor people a single loaf of bread, and half pound of chocolate; so that no one is allowed to carry out a mouthful of provisions; but all is submitted to quietly. The anxiety indeed is so great to get out of Town, that even were we obliged to go naked, it would not hinder us. But there are so many obstructions thrown in the way, that I do not think, those who are most anxious will be all out in less than two or three months — vastly different from what was expected, for the General at first proposed, unasked, to procure the Admiral’ s boats to assist the inhabitants in the transportation of their effects, which is not done, and there are but two ferry-boats allowed to cross. They have their designs in this, which you may easily guess at. We suffer much for want of fresh meat.

“After several months, food shortages in Boston convinced Gage to allow easier emigration from the city,” (American Bar Association, 2012).
In the end, it was the “seizure of these arms from the peaceable citizens of Boston who were not even involved in hostilities,” which ultimately “sent a message to all of the colonies that fundamental rights were in grave danger” (Halbrook, 20008).
Citations:

• “How the British Gun Control Program Precipitated the American Revolution”, 6 Charleston Law Review, 283 (2012), Retrieved at https://papers.ssrn.com/sol3/papers.cfm?abstract_id=1967702
• “History of The Siege Of Boston And Of The Battles Of Lexington, Concord, And Bunker Hill”, Richard Frothingham, 94 (Boston, Charles C. Little & James Brown 1849)
•  “May 21, 1775: General Gage has broken his engagement with the People of Boston, after they had delivered up their Arms.”, The New England Historical Society, 2014. Retrieved at http://www.newenglandhistoricalsociety.com/may-21-1775-general-gage-broken-engagement-people-boston-delivered-arms/
•  “The American Revolution against British Gun Control”, Administrative and Regulatory Law News (American Bar Association). Vol. 37, no. 4, Summer 2012. Retrieved at http://www.davekopel.org/2A/LawRev/american-revolution-against-british-gun-control.html

• “The Founder’s Second Amendment: Origins of the Right to Bear Arms”, Stephen P. Halbrook, 2008.