At its height the Ottoman (Turkish) Empire, centred in Istanbul, stretched from the Crimea to North Africa, but by the mid-nineteenth century it was crumbling. Russia had gained significant areas north of the Black Sea, including the Crimea (now part of the Ukraine), in the years around 1800. In June 1853 Tsar Nicholas I invaded the Ottoman territories to the west of the Black Sea: the principalities of Moldavia and Wallachia, outlined in green on the map. After failed international attempts at diplomacy, Turkey declared war on Russia on 4 October 1853.