Tsar Alexei I was the second Romanov ruler of Russia, reigning from 1645 until his death in 1676. He was the son of Tsar Michael I and is known for his efforts to modernize Russia and strengthen its central authority. His reign saw the expansion of Russian territory and the establishment of a more organized government. Alexei I also faced significant challenges, including internal strife and the Cossack Rebellion. He implemented reforms in the military and administration, and his rule marked the beginning of a more structured Russian state. His legacy includes the promotion of Orthodox Christianity and cultural development.