Cossacks were members of several peasant groups of Russian and Polish descent. They lived in autonomous communal settlements, especially in the Ukraine, until the early 20th century. In return for special privileges, they served in the cavalry under the czars. They were well known for their horsemanship. They raided villages for supplies, women and young men to increase or replenish their ranks. Eventually they became a part of the Russian army.
(From the author's website)
“You're obstinate, little man. You'll make a fine Cossack.”