On 15 August 1770, aged 16, von Kalb became a lieutenant in the Royal Deux-Ponts Regiment of the French Army.
His regiment formed part of the 5,000-strong force sent by the Kingdom of France to fight for the United States in the American Revolutionary War.
On his return to Germany his brother forced him to marry Charlotte von Marschalk und Ostheim.
Since he was soldiering for the rest of the year he could only spend the winter months with his wife and so the marriage proved an unhappy one, with her taking refuge in affairs with Schiller and later with Jean Paul.
Heinrich retired to his estate near Trabelsdorf, where he began an affair with his cook, Barbara Tod, a teacher's daughter, with whom he had three children.