fortification lines across Southern Russia, such as the Great Abatis Line, constantly narrowed the area accessible to incursions.
The death of Ivan's sons marked the end of the ancient Rurik Dynasty in 1598, and in combination with the famine of 1601–03 led to the civil war, the rule of pretenders and foreign intervention during the Time of Troubles in the early 17th century. Polish-Lithuanian Commonwealth occupied parts of Russia, including Moscow. In 1612 the Poles were forced to retreat by the Russian volunteer corps, led by two national heroes, merchant Kuzma Minin and Prince Dmitry Pozharsky. The Romanov Dynasty acceded the throne in 1613 by the decision of Zemsky Sobor, and the country started its gradual recovery from the crisis.
Russia continued its territorial growth through the 17th century, which was the age of Cossacks. Cossacks were warriors organized into military communities, resembling pirates and pioneers of the New World. In 1648, the peasants of Ukraine joined the Zaporozhian Cossacks in rebellion against Poland-Lithuania during the Khmelnytsky Uprising, because of the social and religious oppression they suffered under Polish rule. In 1654 the Ukrainian leader, Bohdan Khmelnytsky, offered to place Ukraine under the protection of the Russian Tsar, Aleksey I. Aleksey's acceptance of this offer led to another Russo-Polish War (1654–1667). Finally, Ukraine was split along the Dnieper River, leaving the western part (or Right-bank Ukraine) under Polish rule and eastern part (Left-bank Ukraine and Kiev) under Russian. Later, in 1670–71 the Don Cossacks led by Stenka Razin initiated a major uprising in the Volga Region, but the Tsar's troops were successful in defeating the rebels.
In the east, the rapid Russian exploration and colonisation of the huge territories of Siberia was led mostly by Cossacks hunting for valuable furs and ivory. Russian explorers pushed eastward primarily along the Siberian River Routes, and by the
