The Hessians were German mercenary soldiers hired by Great Britain to fight in the American Revolutionary War. After the war ended, thousands of Hessian soldiers opted to settle in the United States.
It was common practice among some European monarchs to rent out armies to other leaders when they weren't needed in their home countries. The Landgrave of Hesse-Kassel, Frederick II, was particularly notorious for this. He enlisted as much as 7 percent of his country's population into the military, and "expendable people" such as the unemployed, bankrupt people and servants without masters were subject to mandatory drafting. Once in the military, tight discipline and generous rewards for good performance gave even the most reluctant draftee incentive to do well. Frederick II used the funds from renting his mercenary army out to keep taxes low and services high in his country.
However, the Hessians had a taste for plunder, and they did not limit it to those they fought. Over the years of war, well-publicized instances of Hessians looting innocent towns and even crown loyalists turned sympathies against the English, working as a recruiting and fundraising tool for George Washington's army. Even though only a portion of the over 30,000 German troops who fought in the war were actually Hessians, they were so notorious and feared that all German soldiers in the war became known as such.