Images of the Crimean War, taken by one of the first war photographers, Robert Fenton, starting in 1855 (see entire gallery here).
As the Ottoman Empire declined, Russian territorial ambitions increased. The desire for a warm water naval port pushed Nicholas I to propose partitioning the Ottoman Empire with the British. However, the British, along with other western powers, were interested in maintaining the balance of power in Europe by checking Russian expansionism. The Russian stronghold at Sevastopol became the ultimate goal of the conflict, as the Russian army could directly threat the Ottomans from its strategic position. The allies besieged the city for 11 months, where the allied army experienced a harsh winter with widespread disease and supply shortages. Heavy casualties crippled each side, but the Russians forfeited in September 1855 and exited the war in early 1856.
With the Russian threat distinguished, the once somewhat pro-Ottoman west now experienced growing disdain for the empire. The British–no longer united with the Ottomans against a common foe–had no interest in defending the empire again. Furthermore, the Ottomans maintained territorial interests in the Arabian peninsula and the British saw this encroachment as a challenge to its own ambitions in the region. (Paraphrased from A Brief History of the Late Ottoman Empire by M. Sukru Hanioglu, pp. 79-81)