The Battle of Long Island left the American army in extreme danger. They were crowded into their fortifications at Brooklyn; in front of them was the British and Hessian army, and behind them was the East River. The British and Hessians outnumbered them, and they were steadily digging trenches toward the American lines: once they got close enough, they would attack, and they would almost certainly win. To make things worse, many of the men were exhausted, and it was raining steadily.
If the Americans could cross the East River to New York City, they would be relatively safe, at least for the moment. But the East River was about half a mile wide at this point, and British warships could easily block it: the only thing stopping them at the moment was a strong northeast wind, and that could change at any time. Also, if the enemy troops on land saw the Americans starting to retreat, they would be sure to attack immediately.
The only way was to secretly move the whole army — about 9,000 men, plus as much of their supplies and equipment as possible — across the water during a single night. That seemed nearly impossible under the circumstances, but it was better than the alternative: stay there and wait to be captured or killed.
I’ll let Benjamin Tallmadge, an American officer, tell the story:
After sustaining incessant fatigue and constant watchfulness for two days and nights, attended by heavy rain, exposed every moment to an attack from a vastly superior force in front, and to be cut off from the possibility of retreat to New York, by the fleet, which might enter the East River, on the night of the 29th of August, Gen. Washington commenced recrossing his troops from Brooklyn to New York. To move so large a body of troops, with all their necessary appendages, across a river full a mile wide, with a rapid current, in face of a victorious, well disciplined army, nearly three times as numerous as his own, and a fleet capable of stopping the navigation, so that not one boat could have passed over, seemed to present most formidable obstacles. But, in face of these difficulties, the Commander-in-Chief so arranged his business, that on the evening of the 29th, by 10 o’clock, the troops began to retire from the lines in such a manner that no chasm was made in the lines, but as one regiment left their station on guard, the remaining troops moved to the right and left and filled up the vacancies, while Gen. Washington took his station at the ferry, and superintended the embarkation of the troops. It was one of the most anxious, busy nights that I ever recollect, and being the third in which hardly any of us had closed our eyes in sleep, we were all greatly fatigued. As the dawn of the next day approached, those of us who remained in the trenches became very anxious for our own safety, and when the dawn appeared there were several regiments still on duty. At this time a very dense fog began to rise, and it seemed to settle in a peculiar manner over both encampments. I recollect this peculiar providential occurrence perfectly well; and so very dense was the atmosphere that I could scarcely discern a man at six yards’ distance.
When the sun rose we had just received orders to leave the lines, but before we reached the ferry, the Commander-in-Chief sent one of his Aids to order the regiment to repair again to their former station on the lines. Col. Chester immediately faced to the right about and returned, where we tarried until the sun had risen, but the fog remained as dense as ever. Finally, the second order arrived for the regiment to retire, and we very joyfully bid those trenches a long adieu. When we reached Brooklyn ferry, the boats had not returned from their last trip, but they very soon appeared and took the whole regiment over to New York; and I think I saw Gen. Washington on the ferry stairs when I stepped into one of the last boats that received the troops. …
In the history of warfare I do not recollect a more fortunate retreat. After all, the providential appearance of the fog saved a part of our army from being captured, and certainly myself, among others who formed the rear guard.
The fog lifted soon afterward, the British and Hessians were astonished to find that the Americans had slipped out from under their noses. Some thought that the rebels would soon give up and the war would be over. But Washington’s army wasn’t giving up: they had retreated so that they would be able to fight another day.