In response to the request of the Continental Congress, the Lower Counties Assembly raised the 1st Delaware Regiment, placing Haslet at its command on January 19, 1776, with the rank of colonel. Known as the "Delaware Continentals" or "Delaware Blues", they were from the smallest state, but at some 800 men, were the largest battalion in the army.

At the Battle of Long Island, the Delaware Regiment fought with Colonel William Smallwood's Marylanders. Many thought these were the two best regiments in the Continental Army. They fought under the command of Brigadier General William Alexander, Lord Stirling, and were responsible for holding the Gowanus Road, the far right of the Continental Army line. Retreating across Westchester County, Haslet's men won a victory over a corps of Loyalists at Mamaroneck, New York. At White Plains, on October 28, 1776, the Delaware Regiment again fought with Colonel William Smallwood's Marylanders, reinforcing militia placed on the strategic Chatterton's Hill. The local militia fled under the British attack, but Haslet and Smallwood fought on until, at last, they too yielded the ground.

With expiring enlistments leaving fewer than 100 men remaining in his regiment, Haslet crossed the Delaware with Washington and joined the attack at the Battle of Trenton on the morning of December 26, 1776. However, on January 3, 1777, in a skirmish at the beginning of the Battle of Princeton, with General Hugh Mercer down and wounded, Haslet tried to rally Mercer's brigade and was himself shot in the head and killed instantly.