Modern Russians descend from early groups of Slavs who settled along the rivers of what are today Ukraine and Russia. Following a period of conquest by the Mongols, a series of Russian rulers began to expand their control over the land we now know as Russia. These emperors, called czars, ruled Russia from 1480 to 1917. In 1917 Vladimir Lenin led the Russian Revolution and set up a communist-style state, which later became known as the Soviet Union. The Soviet Union collapsed in 1991 and Russia again became an independent country. The change to a democracy and a free market economy has been a challenge for Russians. Other challenges for Russia include increased trust and cooperation among the different ethnic groups, and the protection of its land, air, and waterways for future generations.