The Landgraviate of Hesse-Kassel, under Frederick II, uncle to King George III, initially provided over 12,000 soldiers to fight in the Americas. Like their British allies, the Hessians had some difficulty acclimating to North America; the first troops to arrive suffered from widespread illness, and forced a delay in the attack on Long Island. From 1776 on, Hessian soldiers were incorporated into the British Army serving in North America, and they fought in most of the major battles, including those of New York and New Jersey campaign, the Battle of Germantown, the Siege of Charleston, and the final Siege of Yorktown, where about 1,300 Germans were taken prisoner, although various reports indicate that the Germans were in better spirits than their British counterparts.

It has been estimated that Hessen-Kassel contributed over 16,000 troops during the course of the Revolutionary War, of whom 6,500 did not return. Hessian officer (later General) Adam Ludwig Ochs estimated that 1,800 Hessian soldiers were killed, but many in the Hessian army intended on staying in America, and remained after the war. Because the majority of German troops came from Hessen, Americans sometimes refer to all German troops generically as "Hessians". Hessen-Kassel signed a treaty of alliance with Great Britain to supply fifteen regiments, four grenadier battalions, two jäger companies, and three companies of artillery. The jägers in particular were carefully recruited and well paid, well clothed, and free from manual labor. These jägers proved essential in the "Indian style" warfare in America, and Great Britain signed a new treaty in December 1777 in which Hesse-Cassel agreed to increase their number from 260 to 1,066.

German armies could not quickly replace the men lost on the other side of the Atlantic, so the Hessians recruited African-Americans as servants and soldiers. 115 black soldiers served with Hessian units, most of them as drummers or fifers.

Perhaps the best-known officer from Hessen-Kassel is General Wilhelm von Knyphausen, who commanded his troops in several major battles. Other notable officers include Colonel Carl von Donop (mortally wounded at the Battle of Red Bank in 1777) and Colonel Johann Rall, who was mortally wounded at the Battle of Trenton in 1776. Rall's regiment was captured, and many of the soldiers were sent to Pennsylvania to work on farms.