Tsar Nicholas I of Russia, based on his role in suppressing the revolutions of 1848 and his mistaken belief that he has British diplomatic support, moves against the Ottomans, who declare war in 1853.

Thus the Crimean War begins, but the European powers are frightened of Russia, and in 1854 Britain, France, and Sardinia join the Ottoman Empire against Russia.

Austria offers the Ottomans diplomatic support, while Prussia remains neutral.

The European allies land in Crimea and lay siege to a well-fortified base at Sevastopol'.

After a year's siege the base falls, exposing Russia's inability to defend a major fortification on its own soil.

Nicholas I dies before the fall of Sevastopol', but even before then he had recognized the failure of his regime.

Russia now has to initiate major reforms or cease to be a competitive major power.