The Battle of Long Island, also known as the Battle of Brooklyn and the Battle of Brooklyn Heights, was an action of the American Revolutionary War fought on August 27, 1776, at and near the western edge of Long Island in present-day Brooklyn. The British defeated the Continental Army and gained access to the strategically important Port of New York, which they held for the rest of the war. It was the first major battle to take place after the United States declared its independence on July 4, 1776, in Philadelphia. It was the largest battle of the Revolutionary War in terms of both troop deployment and combat.

After defeating the British in the siege of Boston on March 17, Continental Army commander-in-chief George Washington relocated his army to defend the port city of New York, located at the southern end of Manhattan Island. Washington understood the city's harbor would provide an excellent base for the Royal Navy, so he established defenses there and waited for the British to attack. In July, the British, under the command of General William Howe, landed a few miles across the harbor on the sparsely populated Staten Island, where they were reinforced by a fleet of ships in Lower New York Bay over the next month and a half, bringing their total force to 32,000 troops. Washington knew the difficulty in holding the city with the British fleet in control of the entrance to the harbor at the Narrows, and moved the bulk of his forces to Manhattan, believing it would be the first target.

On August 21, the British landed on the shores of Gravesend Bay in southwest Kings County, across the Narrows from Staten Island and more than a dozen miles south of the established East River crossings to Manhattan. After five days of waiting, the British attacked the American defenses on Guan Heights. Unbeknownst to the Americans, Howe had brought his main army around their rear and attacked their flank soon after. The Americans panicked, resulting in twenty percent losses through casualties and capture, although a stand by 400 Maryland and Delaware troops prevented greater losses.

The remainder of the army retreated to the main defenses on Brooklyn Heights. The British dug in for a siege, but on the night of August 29–30, Washington evacuated the entire army to Manhattan without the loss of supplies or a single life. The Continental Army was driven out of Manhattan entirely after several more defeats and was forced to retreat through New Jersey to Pennsylvania.

In the first stage of the war, the British Army was trapped in the peninsular city of Boston and was forced to abandon it on March 17, sailing to Halifax, Nova Scotia, to await reinforcements. Washington then began to transfer regiments to New York City, which he believed the British would attack next because of the port's strategic importance. He had sent his second-in-command Charles Lee to New York the previous February to establish the city's defenses. Lee remained in New York City until March 7 when the Continental Congress sent him to the Carolinas.. Before he departed for the South, Lee had also seen to it that the immediate area was cleared of Loyalists. After Lee's departure, construction of the city's defenses was left to American General William Alexander, Lord Stirling.

Washington left Boston on April 4, arrived in New York on April 13, and established headquarters at the former home of Archibald Kennedy on Broadway facing Bowling Green. Troops were in limited supply, so Washington found the defenses incomplete, but Lee had concluded in any case it would be impossible to hold the city with the British commanding the sea. He reasoned the defenses should be located with the ability to inflict heavy casualties upon the British if any move was made to take and hold ground. Barricades and redoubts were established in and around the city, and the bastion of Fort Stirling was built across the East River in Brooklyn Heights, facing the city.

Washington began moving troops to Brooklyn in early May, and there were several thousand of them there in a short time. Three more forts were under construction on the eastern side of the East River to support Fort Stirling, which stood to the west of the hamlet of Brooklyn Heights. These new fortifications were Fort Putnam, Fort Greene, and Fort Box (named for Major Daniel Box). They lay from north to south, with Fort Putnam farthest to the north, Greene slightly to the southwest, and Box slightly farther southwest. Each of these defensive structures was surrounded by a large ditch, all connected by a line of entrenchments and a total of 36 cannons.

Fort Defiance was also being constructed at this time, located farther southwest past Fort Box, near present-day Red Hook. In addition to these new forts, a mounted battery was established on Governors Island, cannons were placed at Fort George facing Bowling Green on Manhattan, and more cannons were placed at the Whitehall Dock, which sat on the East River. Hulks were sunk at strategic locations to deter the British from entering the East River and other waterways.