After the Boston Tea Party, an extremely angry Parliament passed what was called The Intolerable (or Coercive) Acts, intended to punish the city of Boston for the acts of the Tea Partiers and enforced by the British military. The British governor of Massachusetts, General Thomas Gage, tried to disperse a town meeting in Salem but the troops he sent were forced to retreat when 3000 armed Americans responded. According to John Andres, Gage's aide, everyone in the area over 16 years of age owned and was experienced with a gun and owned plenty of gunpowder.
Realizing that it would be impossible to enforce the new laws on a well-armed populace with only 2000 troops in Boston. Gage decided to send men to Charlestown to capture the powderhouse - an important building where the members of a community would store their gunpowder to keep the volatile explosive away from their homes. The British seized hundreds of barrels of gunpowder, provoking over 20,000 militiamen from the surrounding areas to start marching toward Boston (but dispersed upon learning that there was no fighting in the city).
Soon after, General Gage began warrantless searches and seizures of firearms and ammunition throughout Boston. The Boston Gazette reported that of all of Gage's policies, "what most irritated the People" was "seizing their Arms and Ammunition." To reduce the supply of guns in America, on October 19, 1774, the British started an arms embargo of America that required British subjects to have a permit to export guns and ammo to America, while simply not issuing any such permits.
In December of 1774, New Hampshire militiamen pre-emptively captured Fort William and Mary, near Portsmouth, and all the arms stores in it, upon learning that the British had sent two warships to do the same.
As a result of the outrage over the arms embargo and other gun control measures, many Americans started to form militias that were independent of the British government.
Suddenly, it happened. On April 19, 1775, 700 British Redcoats under Major Pitcairn left the city of Boston with the objective of seizing stores of American weapons at Lexington and Concord. Paul Revere warned the militias of the approaching British troops, and 200 men, aged from 16 to 60, were gathered to meet the British at Lexington. The Americans were quickly beaten, and the British advanced to Concord where the they believed the local Patriots kept most of their arms (including 2 cannons).
But the Americans, knowing that the British were coming to confiscate their weapons, secretly hauled off their arms stores to safety. A group of militia managed to defeat some of the British force at Concord's North Bridge. Disappointed in what they didn't find, the British began to retreat back to Boston, and that's when the massacre really happened.
Hundreds of armed Americans started swarming in from nearby towns to help; they soon doubled the numbers of the British as the Redcoats were peppered with surprisingly accurate rifle and musket fire from opportunist snipers for the entirety of their retreat. The Americans were running out of ammo and powder by the time British reinforcements from Boston came to escort what was left of Major Pitcairn's troops.
That night, American militiamen began the siege of Boston. The War for Independence had started, and it started in response to gun control.