The Battles of Lexington and Concord were the first military engagements of the American Revolutionary War. The battles were fought on April 19, 1775, in Middlesex County, Province of Massachusetts Bay, within the towns of Lexington, Concord, Lincoln, Menotomy (present-day Arlington), and Cambridge. They marked the outbreak of armed conflict between the Kingdom of Great Britain and its thirteen colonies in America.

In late 1774, Colonial leaders adopted the Suffolk Resolves in resistance to the alterations made to the Massachusetts colonial government by the British parliament following the Boston Tea Party. The colonial assembly responded by forming a Patriot provisional government known as the Massachusetts Provincial Congress and calling for local militias to train for possible hostilities. The Colonial government exercised effective control of the colony outside of British-controlled Boston. In response, the British government in February 1775 declared Massachusetts to be in a state of rebellion.

About 700 British Army regulars in Boston, under Lieutenant Colonel Francis Smith, were given secret orders to capture and destroy Colonial military supplies reportedly stored by the Massachusetts militia at Concord. Through effective intelligence gathering, Patriot leaders had received word weeks before the expedition that their supplies might be at risk and had moved most of them to other locations. On the night before the battle, warning of the British expedition had been rapidly sent from Boston to militias in the area by several riders, including Paul Revere and Samuel Prescott, with information about British plans. The initial mode of the Army's arrival by water was signaled from the Old North Church in Boston to Charlestown using lanterns to communicate "one if by land, two if by sea".

The first shots were fired just as the sun was rising at Lexington. Eight militiamen were killed, including Ensign Robert Munroe, their third in command. The British suffered only one casualty. The militia was outnumbered and fell back, and the regulars proceeded on to Concord, where they broke apart into companies to search for the supplies. At the North Bridge in Concord, approximately 400 militiamen engaged 100 regulars from three companies of the King's troops at about 11:00 am, resulting in casualties on both sides. The outnumbered regulars fell back from the bridge and rejoined the main body of British forces in Concord.

The British forces began their return march to Boston after completing their search for military supplies, and more militiamen continued to arrive from the neighboring towns. Gunfire erupted again between the two sides and continued throughout the day as the regulars marched back towards Boston. Upon returning to Lexington, Lt. Col. Smith's expedition was rescued by reinforcements under Brigadier General Hugh Percy, a future Duke of Northumberland styled at this time by the courtesy title Earl Percy. The combined force of about 1,700 men marched back to Boston under heavy fire in a tactical withdrawal and eventually reached the safety of Charlestown. The accumulated militias then blockaded the narrow land accesses to Charlestown and Boston, starting the siege of Boston.

Ralph Waldo Emerson describes the first shot fired by the Patriots at the North Bridge in his "Concord Hymn" as the "shot heard round the world".

The American Revolution was an ideological and political revolution that occurred in British America between 1765 and 1791. The Americans in the Thirteen Colonies formed independent states that defeated the British in the American Revolutionary War (1775–1783), gaining independence from the British Crown, establishing the constitution that created the United States of America, the first modern constitutional liberal democracy.American colonists objected to being taxed by the British Parliament, a body in which they had no direct representation. Before the 1760s, Britain's American colonies had enjoyed a high level of autonomy in their internal affairs, which were locally governed by colonial legislatures. The passage of the Stamp Act of 1765 imposed internal taxes on official documents and newspapers printed in the colonies, which led to colonial protest, and the meeting of representatives of several colonies in the Stamp Act Congress. Tensions relaxed with the British repeal of the Stamp Act, but flared again with the passage of the Townshend Acts in 1767. The British government deployed troops to Boston in 1768 to quell unrest, leading to the Boston Massacre in 1770. The British government repealed most of the Townshend duties in 1770, but retained the tax on tea in order to symbolically assert Parliament's right to tax the colonies. The burning of the Gaspee in Rhode Island in 1772, the passage of the Tea Act of 1773 and the Boston Tea Party in December 1773 led to a new escalation in tensions. The British responded by closing Boston Harbor and enacting a series of punitive laws which effectively rescinded Massachusetts Bay Colony's privileges of self-government. The other colonies rallied behind Massachusetts, and twelve of the thirteen colonies sent delegates in late 1774 to form a Continental Congress for the coordination of their resistance to Britain. Opponents of Britain were known as Patriots or Whigs, while colonists who retained their allegiance to the Crown were known as Loyalists or Tories.

Open warfare erupted when British regulars sent to capture a cache of military supplies were confronted by local Patriot militia at Lexington and Concord on April 19, 1775. Patriot militia, joined by the newly formed Continental Army, then put British forces in Boston under siege by land and their forces withdrew by sea. Each colony formed a Provincial Congress, which assumed power from the former colonial governments, suppressed Loyalism, and contributed to the Continental Army led by Commander in Chief General George Washington. The Patriots unsuccessfully attempted to invade Quebec and rally sympathetic colonists there during the winter of 1775–76.

The Continental Congress declared British King George III a tyrant who trampled the colonists' rights as Englishmen, and they pronounced the colonies free and independent states on July 4, 1776. The Patriot leadership professed the political philosophies of liberalism and republicanism to reject rule by monarchy and aristocracy. The Declaration of Independence proclaimed that all men are created equal, though it was not until later centuries that constitutional amendments and federal laws would increasingly grant equal rights to African Americans, Native Americans, poor white men, and women.

The British captured New York City and its strategic harbor in the summer of 1776, which they held for the duration of the war. The Continental Army captured a British army at the Battle of Saratoga in October 1777, and France then entered the war as an ally of the United States, expanding the war into a global conflict. The British Royal Navy blockaded ports and captured New York City for the duration of the war, and other cities for brief periods, but they failed to destroy Washington's forces. Britain also attempted to hold the Southern states with the anticipated aid of Loyalists, and the war moved south. British general Charles Cornwallis captured an American army at Charleston, South Carolina in early 1780, but he failed to enlist enough volunteers from Loyalist civilians to take effective control of the territory. Finally, a combined American and French force captured Cornwallis' army at Yorktown in the fall of 1781, effectively ending the war. The Treaty of Paris was signed on September 3, 1783, formally ending the conflict and confirming the new nation's complete separation from the British Empire. The United States took possession of nearly all the territory east of the Mississippi River and south of the Great Lakes, with the British retaining control of northern Canada, and Spain taking back Florida.

Among the significant results of the war were American independence and the end of British mercantilism in America, opening up worldwide trade for the United States - including with Britain. Around 60,000 Loyalists migrated to other British territories, particularly to Canada, but the great majority remained in the United States. The Americans soon adopted the United States Constitution, replacing the weak wartime Confederation and establishing a comparatively strong national government structured as a federal republic, which included an elected executive, a national judiciary, and an elected bicameral Congress representing states in the Senate and the population in the House of Representatives. It is the world's first federal democratic republic founded on the consent of the governed. Shortly after a Bill of Rights was ratified as the first ten amendments, guaranteeing fundamental rights used as justification for the revolution.