Russian Empire
Though the Empire was only officially proclaimed by Tsar Peter I following the Treaty of Nystad (1721), some historians would argue that it was truly born either when Ivan III of Russia conquered Veliky Novgorod in 1478, or when Ivan the Terrible conquered the Khanate of Kazan in 1552. According to another point of view, the term "Tsardom", which was used after the coronation of Ivan IV in 1547, was already a contemporary Russian word for empire.
Russia
Russia's territorial expansion was achieved largely in the late 16th century under the Cossack Yermak Timofeyevich during the reign of Ivan the Terrible, at a time when competing city-states in the western regions of Russia had banded together to form one country. Yermak mustered an army and pushed eastward where he conquered nearly all the lands once belonging to the Mongols, defeating their ruler, Khan Kuchum.