After a troubled 14-year regency, Vasilii III’s son and heir, Ivan IV, became the first Muscovite tsar (cesar) in 1547. The imperial implications of his title proved increasingly apt during the following extraordinary half-century, as Moscow was ruled by Ivan and his son, Fedor (Theodore). Muscovy acquired still more territory, more ethnically and religiously diverse populations, and confronted the new military challenges associated with imperial and territorial aggrandizement.