When the first skirmishes of the Revolutionary War broke out in Massachusetts in April 1775, few people in the American colonies wanted to separate from Great Britain entirely. But as the war continued, and Britain called out massive armed forces to enforce its will, more and more colonists came to accept that asserting independence was the only way forward.

And the Declaration of Independence would play a critical role in unifying the colonies from the bloody struggle they now faced.

Over the decade following the passage of the Stamp Act in 1765, a series of unpopular British laws met with stiff opposition in the colonies, fueling a bitter struggle over whether Parliament had the right to tax the colonists without the consent of the representative colonial governments. This struggle erupted into violence in 1770 when British troops killed five colonists in the Boston Massacre.

Three years later, outrage over the Tea Act of 1773 prompted colonists to board an East India Company ship in Boston Harbor and dump its cargo into the sea in the now-infamous Boston Tea Party.

In response, Britain cracked down further with the Coercive Acts, going so far as to revoke the colonial charter of Massachusetts and close the port of Boston. Resistance to the Intolerable Acts, as they became known, led to the formation of the First Continental Congress in Philadelphia in 1774, which denounced “taxation without representation” - but stopped short of demanding independence from Britain.

Then the first shots rang out between colonial and British forces at Lexington and Concord, and the Battle of Bunker Hill cost hundreds of American lives, along with 1,000 killed on the British side.

Some 20,000 troops under General George Washington faced off against a British garrison in the Boston Siege, which ended when the British evacuated in March 1776. Washington then moved his Continental Army to New York, where he assumed (correctly) that a major British invasion would soon take place.

Meanwhile, many in the Continental Congress still clung to the assumption that reconciliation with Britain was the ultimate goal. This would soon change, thanks in part to the actions of King George III, who in October 1775 denounced the colonies in front of Parliament and began building up his army and navy to crush their rebellion.

In order to have any hope of defeating Britain, the colonists would need support from foreign powers (especially France), which Congress knew they could only get by declaring themselves a separate nation.

In his bestselling pamphlet, “Common Sense,” a recent English immigrant named Thomas Paine also helped push the colonists along on their path toward independence.

“His argument was that we had to break from Britain because the system of the British constitution was hopelessly flawed,” the late Pauline Maier, professor of history at the Massachusetts Institute of Technology (MIT), said in a 2013 lecture on “The Making of the Declaration of Independence.”

“[Britain] had hereditary rule, it had kings—you could never have freedom so long as you had hereditary rule.”

After Virginia delegate Richard Henry Lee introduced a motion to declare independence on June 7, 1776, Congress formed a committee to draft a statement justifying the break with Great Britain.

The initial draft of the Declaration of Independence was written by Thomas Jefferson and was presented to the entire Congress on June 28 for debate and revision.

In addition to Jefferson’s eloquent preamble, the document included a long list of grievances against King George III, who was accused of committing many “injuries and usurpations” in his quest to establish “an absolute tyranny over these States.”

After two days of editing and debate, the Congress adopted the Declaration of Independence on July 4, 1776, even as a large British fleet and more than 34,000 troops prepared to invade New York. By the time it was formally signed on August 2, printed copies of the document were spreading around the country, being reprinted in newspapers and publicly read aloud.

While the road to independence had been long and twisted, the effect of its declaration made an impact right away.

“It changed the whole character of the war,” Maier said. “These were people who for a year had been making war against a king with whom they were trying to effect a reconciliation, to whom they were publicly professing loyalty. Now heart and hand, as one person said, could move together. They had a cause to fight for.”