The militarist and nationalistic reign of Czar Nicholas I (1825–1855) led to wars of conquest against Persia (1826–1828) and Turkey (1828–1829). Various rebel tribes in the Caucasus region were crushed. A Polish revolt in 1830 was ruthlessly crushed. Russian troops in 1848 crossed into Austria-Hungary to put down the Hungarian Revolt. Russification policies were implemented to weaken minority ethnic groups. Nicholas also built the Kremlin Palace and a new cathedral in Saint Petersburg, and Pan-Slavist ambition led to further war with Turkey (the sick man of Europe) in 1853 provoked Britain and France into invading Crimea, and Nicholas died supposedly of grief at his defeat.