 He is initially documented by references in the 1620s to his position as the Cossack Polkovnyk (Colonel) Hassan Tarasa in Hungarian chronicles, noted for considerable cruelty during his participation in the Thirty Year War as a Habsburg mercenary.In 1629, after the pro-Polish Cossack Hetman Mykhailo Doroshenko was killed in Crimea, the unregistered Cossacks elected Fedorovych to the Hetmanship and, under his leadership, participated in the subsequent Crimean campaign.In March 1630, Fedorovych became the leader of a Cossack and peasant revolt, which became known as the "Fedorovych Uprising" carried out by the unregistered Cossacks dissatisfied with the conditions of the 1625 Treaty of Kurukove (also known as the Treaty of Lake Kurukove) signed earlier by Doroshenko, which restricted the number of the registry to only six thousand.