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 Crimea, 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.