Kościuszko was born in February 1746 in a manor house on the
Mereczowszczyzna estate near
Kosów in
Nowogródek Voivodeship, Grand Duchy of Lithuania, a part of the
Polish–Lithuanian Commonwealth. His exact birthdate is unknown; commonly cited are 4 February and 12 February. Kościuszko was the youngest son of a member of the
szlachta (untitled Polish nobility), , an officer in the
Polish–Lithuanian Commonwealth Army, and his wife . The family held the Polish
Roch III coat of arms. At the time of Tadeusz Kościuszko's birth, the family possessed modest landholdings in the Grand Duchy worked by 31 peasant families. Tadeusz was baptized in the
Catholic church, thereby receiving the names
Andrzej,
Tadeusz, and
Bonawentura. His paternal family was originally
Ruthenian and traced their ancestry to Konstanty Fiodorowicz Kostiuszko, a courtier of
Polish King and
Grand Duke of Lithuania Sigismund I the Old. Kościuszko's maternal family, the Ratomskis, were also Ruthenian. , where Kościuszko attended the
Corps of Cadets His family had become
Polonized as early as the 16th century. Like most Polish–Lithuanian nobility of the time, the Kościuszkos spoke
Polish and identified with
Polish culture. Kościuszko also, as was common for Polish nobility in the region, clearly stressed his attachment to the multiethnic Identity of the Grand Duchy of Lithuania in later letters. For example, in 1790 Kościuszko wrote "If this does not soften you and you do not raise my case in the Sejm so that I can return, I myself will probably, God sees, do something bad to myself, as I am angry because being from Lithuania I serve the Kingdom [of Poland] when you do not have three generals", while during the Uprising of 1794 Kościuszko wrote "Lithuania! My countrymen and tribesmen! I was born in your land, sincere love for my homeland evokes in me a special favor for those among whom I began my life". and after graduating on 20 December 1766, Kościuszko was promoted to
chorąży, a military rank roughly equivalent to modern
lieutenant. He stayed on as a student instructor and, by 1768, had attained the rank of
captain. Kościuszko did not give up on improving his military knowledge. He audited lectures for five years and frequented the libraries of the Paris military academies. His exposure to the French
Enlightenment, along with the
religious tolerance practised in the Polish–Lithuanian Commonwealth, strongly influenced his later career. The French economic theory of
physiocracy made a particularly strong impression on his thinking. He also developed his artistic skills, and while his career took him in a different direction, all his life he continued drawing and painting. In the
First Partition of the Polish–Lithuanian Commonwealth in 1772,
Russia,
Prussia, and
Austria annexed large swaths of Commonwealth territory and gained influence over the internal politics. When Kościuszko returned home in 1774, he found that his brother Józef had squandered most of the family fortune, and there was no place for him in the Army, as he could not afford to buy an officer's commission.