all these conquests were returned to the Kingdom of Prussia by pro-Prussian Peter III of Russia. Catherine II ("the Great"), who ruled in 1762–1796, presided over the Russian Age of Enlightenment. She extended Russian political control over the Polish–Lithuanian Commonwealth and annexed most of its territories into Russia, making it the most populous country in Europe. In the south, after the successful Russo-Turkish Wars against the Ottoman Empire, Catherine advanced Russia's boundary to the Black Sea, by dissolving the Crimean Khanate, and annexing Crimea. As a result of victories over Qajar Iran through the Russo-Persian Wars, by the first half of the 19th century, Russia also conquered the Caucasus. Catherine's successor, her son Paul, was unstable and focused predominantly on domestic issues. Following his short reign, Catherine's strategy was continued with Alexander I's (1801–1825) wresting of Finland from the weakened Sweden in 1809, and of Bessarabia from the Ottomans in 1812. In North America, the Russians became the first Europeans to reach and colonise Alaska. In 1803–1806, the first Russian circumnavigation was made. In 1820, a Russian expedition discovered the continent of Antarctica. During the Napoleonic Wars, Russia joined alliances with various European powers, and fought against France. The French invasion of Russia at the height of Napoleon's power in 1812 reached Moscow, but eventually failed miserably as the obstinate resistance in combination with the bitterly cold Russian winter led to a disastrous defeat of invaders, in which the pan-European Grande Armée faced utter destruction. Led by Mikhail Kutuzov and Michael Andreas Barclay de Tolly, the Imperial Russian Army ousted Napoleon and drove throughout Europe in the War of the Sixth Coalition, ultimately entering Paris. Alexander I controlled Russia's delegation at the Congress of Vienna, which defined the map of post-Napoleonic Europe. The officers who pursued Napoleon into Western Europe brought ideas of liberalism back to Russia, and attempted to curtail the tsar's powers during the abortive Decembrist revolt of 1825. At the end of the conservative reign of Nicholas I (1825–1855), a zenith period of Russia's power and influence in Europe, was disrupted by defeat in the Crimean War. Nicholas's successor Alexander II (1855–1881) enacted significant changes throughout the country, including the emancipation reform of 1861. These reforms spurred industrialisation, and modernised the Imperial Russian Army, which liberated much of the Balkans from Ottoman rule in the aftermath of the 1877–1878 Russo-Turkish War. During most of the 19th and