The most important military confrontation during the reign of Sultan Abdülmecid was the three-year Crimean War (1853-1856) between the Russians and the British, French, Sardinians and Ottomans. It marks the first Ottoman war, for which Ottomans allied with other countries. Among the major reasons of the war was that Russians desired to expand their territory south to the Ottoman land, and the British and French wanted to stop them. Allied soldiers came to Istanbul and settled in Selimiye Barracks then temporarily used it as a hospital.
The famous English nurse, Florence Nightingale (1820-1910), dubbed "The Lady with the Lamp", was invited to Istanbul to help the wounded British soldiers during the Crimean War. She arrived in November 1854 at Selimiye Barracks, which was temporarily used as an infirmary. Because of her personal initiative, the death rate among the wounded soldiers was reduced by 90%, which made her a national heroine in Britain. In 1954, to mark the centennial of her arrival in Selimiye, the Turkish Federation of Nurses converted Nightingale's rooms into the Florence Nightingale Museum.
In the Crimean War, one of the young artillery officers in imperial Russia’s military was Leo Tolstoy, a great Russian novelist, and the author of War and Peace (1869) and Anna Karenina (1877). Tolstoy described his experiences during the Siege of Sevastopol in “Sevastopol Sketches”.
The allied army defeated Russians and it became a victory for Abdülmecid too. After the war, the Ottoman Empire fell into a financial crisis. Because of the war, the Empire had to borrow from creditors for the first time in 1854. The growing demand for new loans after the war was beyond the capacity of the Galata Bankers (sarrafs or moneychangers). The Ottoman Bank, the first financial institution, was created in 1856 by a private British investment, later it became a British-French joint corporation.