On August 26, 1776, the Marylanders arrived at Long Island on the eve of battle. Once it became clear that a major engagement was imminent, Washington sent the regiment to reinforce the American defensive line. The men who would become known as the Maryland 400 were posted on the Heights of Guana, a wooded, ten-mile ridge near the British encampment at the town of Flatbush. They joined with the force already there, which had fought a number of skirmishes with the British, and the small engagements served to boost the confidence of the inexperienced Continental soldiers. An intelligence report from New York mentioned the recent encounters with the British, “We have had only four men wounded since the enemy landed; but we are certain many of them [the British] fell.” 
The men of the First Maryland Regiment were sent to Long Island without their commander, Colonel William Smallwood. Smallwood had been ordered by George Washington to sit at the court martial of Colonel Herman Zedwitz, who was accused of attempting to sell American intelligence to the British. Smallwood later wrote that he had “waited on General Washington and urged the necessity of attending our troops, yet he refused to discharge us.” The court martial would continue until late on the 26th, and Smallwood’s men would already be engaged in combat by the time he returned to his regiment on the 27th.
George Washington was also on Long Island on August 26. Although the contemporary documents reveal little about his actions that day, he probably rode with Generals Sullivan and Putnam to the Heights of Guana in order to observe the British encampments below at Flatlands. Despite the lack of reliable intelligence on British numbers and movements, the American leaders realized that the British were planning to fight on Long Island. “We are led to think that they mean to land the main body of their army on Long Island,” Washington wrote to Congress on the 26th, “and to make their grand push there.” However, the Americans did not know that the British had already landed the majority of their army on Long Island. Additionally, the day before, on August 25, two Hessian brigades under Lieutenant General De Heister had landed on the island, adding 5,000 men to the British forces. The roughly 7,000 Americans were dramatically outnumbered by the approximately 20,000 British troops encamped on Long Island.
After sunset on August 26, below the Heights of Guana, the white tents of the British stood empty and their burning campfires were left unattended. 10,000 British soldiers crept into the night to execute a delicate flanking maneuver and surround the Americans posted on the ridge. Three Loyalist farmers led the the way for the column of troops, which stretched more than two miles. Under the command of Generals Cornwallis and Clinton, they followed the sparsely guarded Jamaica Road, taking all five American cavalrymen picketing the road captive during the night march. By the next morning, the British had walked nine miles to the Bedford road, north of the Heights of Guana. The British trap at the Battle of Long Island was set.
The exact numbers of American and British forces are unknown; I have included here an approximate number that is based on the estimates of a number of historians.