During the summer of 1776, a British military force of 32,000 troops and 10,000 sailors on more than 400 ships assembled in Staten Island and the New York Harbor. They were obviously preparing for an attack on New York City at the southern end of York Island (now Manhattan).
General George Washington at his headquarters in the City obviously knew of the assembling adversary forces and of the approaching battle. But he did not know when or where the attack would come. Therefore, Washington split the Continental Army of 19,000 troops into fortified positions in the City and in Long Island across the East River from the City.
In further preparations for the forthcoming battles, the colonial forces that Spring and Summer constructed Fort Stirling on present-day Brooklyn Heights (Long Island) overlooking New York City and four other nearby forts.
On August 22nd, the British made their offensive, tactical decision. They sent 15,000 troops on boats over three miles of water from Staten Island to Long Island. They landed unopposed on the shore of Gravesend Bay. (The red area in the photo to the right is the Narrows which the boats transversed.)
The troops then marched six miles inland to establish camp at the village of Flatbush. (Two days later these forces were augmented by the arrival of 5,000 Hessian troops from Staten Island.)
Washington was aware of the initial British movement into Long Island, but was told it only involved 8,000 to 9,000 troops. Therefore, he thought this was a diversionary move and that the main attack would be on York Island (Manhattan). As a result, Washington only sent an additional 1,500 troops to Long Island to bring the total colonial force to 6,000 at that location.
Five days later, August 27th, the British launched their attack. Its apparent major focus was the center of the American fortifications on the Guanus hills southeast of Brooklyn Heights. Despite the heavy colonial fortifications, the British met with little resistance at this site. In fact, however, the main British contingent advanced on the unguarded Jamaica Pass, which was the eastern-most passage through the Guana Heights.
Washington and his exhausted men fell back that day to their fortifications on Brooklyn Heights, waiting as night fell for a final British assault. Most of the American soldiers were in a trap, facing 20,000 British regulars at their front, and a mile-wide river at their backs. All the British navy had to do was to move a few warships up the East River to prevent Washington’s escape, and the war would be over, the revolution defeated.
But British General Howe did not press the British victory with an immediate pursuit of the retreating colonial forces. Instead, he halted the attack and started preparations for a siege of the American fortifications in Brooklyn Heights and Cobble Hill.
On August 29th, the temperature dropped sharply and the rain came in torrents on the unsheltered American army. This storm prevented five British warships from advancing up the East River to encircle the Americans in Brooklyn Heights. That same day Washington ordered his troops to withdraw in the dead of night across the river to Manhattan. The storm threatened this evacuation, but at about 11:00 p.m. the wind shifted and facilitated the evacuation. John Glover’s Massachusetts sailors and fishermen with every kind of small craft started over the river from New York City to rescue Washington’s army.
Early the next morning of the 30th, many of Washington’s men were still on Long Island. But a fog appeared on the Long Island side of the East River that concealed the final movement of the American troops to Manhattan. The entire maneuver was completed by 7:00 a.m. that morning, having taken 13 hours to ferry 9,000 men, their horses and their equipment across the East River.
Later that morning the British were surprised to discover the colonial forces had escaped. After that discovery, the British occupied the American fortifications on Brooklyn Heights.
When the last shot was fired, 1,200 Americans were dead and another 1,500 wounded, captured or missing. The British suffered a mere 60 dead and 300 wounded or missing. This was the first major battle of the War and what turned out to be the largest battle of the entire War, and it was a disastrous one for the Americans.
My maternal fifth great-grandfather, Perley Brown, from Leicester (Worcester County), Massachusetts was stationed in New York City at this time and thus did not fight in this battle.
 In addition to the hyperlinked sources in this post, it also draws from David McCullough, 1776 at 122-197 (New York; Simon & Schuster 2005); T. Harry Williams, Richard N. Current & Frank Freidel, A History of the United States [To 1876], at 151 (New York: Alfred A. Knopf 1959); Henry Steele Commager & Richard B. Morris, The Spirit of ‘Seventy-Six: The Story of the American Revolution as Told by Participants, Ch. Eleven (New York: Harper & Row, 1967). From July 1966 through March 1970, my family and I lived in Brooklyn Heights where some of the Battle of Brooklyn (Long Island) occurred. As a result, I frequently walked around this area, but unfortunately I did not scout out these historical sites.
 Carol Willits Brown, William Brown–English Immigrant of Hatfield and Leicester, Massachusetts, and His Descendants c. 1669-1994 at 17-23(Gateway Press; Baltimore, MD 1994).