Czar of Russia, born at St. Petersburg, third son of Paul I., ascended the throne in 1825 in succession to Alexander I., his eldest brother; suppressed with rigour and not a little severity a formidable conspiracy which took form on his accession; took up arms against Persia and wrested Erivan from its sway, struggled against both the Poles and the Turks till his overbearing policy against the latter provoked a coalition of France, England, and Sardinia to their defence in the Crimean War, which was still going on when he died; in 1848 he aided Austria in the suppression of the Hungarian insurrection (1796-1855).