The Crimean War was a military conflict fought from October 1853 to February 1856 in which Russia lost to an alliance of the Ottoman Empire, France, the United Kingdom and Piedmont-Sardinia. The immediate cause of the war involved the rights of Christian minorities in Palestine (then part of the Ottoman Empire) with the French promoting the rights of Roman Catholics, and Russia promoting those of the Eastern Orthodox Church. Longer-term causes involved the decline of the Ottoman Empire, the expansion of the Russian Empire in the preceding Russo-Turkish Wars, and the British and French preference to preserve the Ottoman Empire to maintain the balance of power in the Concert of Europe.

The churches worked out their differences with the Ottomans and came to an agreement, but both the French Emperor Napoleon III and the Russian Tsar Nicholas I refused to back down. Nicholas issued an ultimatum that demanded the Orthodox subjects of the Ottoman Empire be placed under his protection. Britain attempted to mediate and arranged a compromise to which Nicholas agreed. When the Ottomans demanded changes to the agreement, Nicholas recanted and prepared for war.

In July 1853, Russian troops occupied the Danubian Principalities (now part of Romania but then under Ottoman suzerainty). In October 1853, having obtained promises of support from France and Britain, the Ottomans declared war on Russia. Led by Omar Pasha, the Ottomans fought a strong defensive campaign and stopped the Russian advance at Silistra (now in Bulgaria). A separate action on the fort town of Kars, in Western Armenia, led to a siege, and an Ottoman attempt to reinforce the garrison was destroyed by a Russian fleet at the Battle of Sinop in November 1853. Fearing an Ottoman collapse, the British and the French had their fleets enter the Black Sea in January 1854. They moved north to Varna in June 1854 and arrived just in time for the Russians to abandon Silistra.

After a minor skirmish at Kstence (now Constana), the allied commanders decided to attack Russia's main naval base in the Black Sea, Sevastopol, on the Crimean Peninsula. After extended preparations, allied forces landed on the peninsula in September 1854 and marched their way to a point south of Sevastopol after they had won the Battle of the Alma on 20 September 1854. The Russians counterattacked on 25 October in what became the Battle of Balaclava and were repulsed, but the British Army's forces were seriously depleted as a result. A second Russian counterattack, at Inkerman (November 1854), ended in a stalemate as well. The front settled into the siege of Sevastopol, involving brutal conditions for troops on both sides. Smaller military actions took place in the Baltic (18541856; see land War), the Caucasus (18531855), the White Sea (JulyAugust 1854) and the North Pacific (18541855).

Sevastopol finally fell after eleven months, after the French had assaulted Fort Malakoff. Isolated and facing a bleak prospect of invasion by the West if the war continued, Russia sued for peace in March 1856. France and Britain welcomed the development, owing to the conflict's domestic unpopularity. The Treaty of Paris, signed on 30 March 1856, ended the war. It forbade Russia from basing warships in the Black Sea. The Ottoman vassal states of Wallachia and Moldavia became largely independent. Christians in the Ottoman Empire gained a degree of official equality, and the Orthodox Church regained control of the Christian churches in dispute.The Crimean War was one of the first conflicts in which military forces used modern technologies such as explosive naval shells, railways and telegraphs. The war was one of the first to be documented extensively in written reports and in photographs. The war quickly became a symbol of logistical, medical and tactical failures and of mismanagement. The reaction in Britain led to a demand for professionalisation of medicine, most famously achieved by Florence Nightingale, who gained worldwide attention for pioneering modern nursing while she treated the wounded.

The Crimean War marked a turning point for the Russian Empire. The war weakened the Imperial Russian Army, drained the treasury and undermined Russia's influence in Europe. The empire would take decades to recover. Russia's humiliation forced its educated elites to identify its problems and to recognise the need for fundamental reforms. They saw rapid modernisation as the sole way to recover the empire's status as a European power. The war thus became a catalyst for reforms of Russia's social institutions, including the abolition of serfdom and overhauls in the justice system, local self-government, education and military service.

The Ottoman Empire (; Ottoman Turkish: دولت عليه عثمانيه Devlet-i ʿAlīye-i ʿOsmānīye, lit. 'The Sublime Ottoman State'; Turkish: Osmanlı İmparatorluğu or Osmanlı Devleti; French: Empire ottoman) was an empire that controlled much of Southeastern Europe, Western Asia, and Northern Africa between the 14th and early 20th centuries. It was founded at the end of the 13th century in northwestern Anatolia in the town of Söğüt (modern-day Bilecik Province) by the Turkoman tribal leader Osman I. After 1354, the Ottomans crossed into Europe and with the conquest of the Balkans, the Ottoman beylik was transformed into a transcontinental empire. The Ottomans ended the Byzantine Empire with the conquest of Constantinople in 1453 by Mehmed the Conqueror.Under the reign of Suleiman the Magnificent, the Ottoman Empire marked the peak of its power and prosperity, as well as the highest development of its governmental, social, and economic systems. At the beginning of the 17th century, the empire contained 32 provinces and numerous vassal states. Some of these were later absorbed into the Ottoman Empire, while others were granted various types of autonomy over the course of centuries. With Constantinople (modern-day Istanbul) as its capital and control of lands around the Mediterranean Basin, the Ottoman Empire was at the centre of interactions between the Middle East and Europe for six centuries.

While the empire was once thought to have entered a period of decline following the death of Suleiman the Magnificent, this view is no longer supported by the majority of academic historians. The newer academic consensus posits that the empire continued to maintain a flexible and strong economy, society and military throughout the 17th and for much of the 18th century. However, during a long period of peace from 1740 to 1768, the Ottoman military system fell behind that of their European rivals, the Habsburg and Russian empires. The Ottomans consequently suffered severe military defeats in the late 18th and early 19th centuries. The successful Greek War of Independence concluded with decolonization of Greece following the London Protocol (1830) and Treaty of Constantinople (1832). This & other defeats prompted the Ottoman state to initiate a comprehensive process of reform and modernization known as the Tanzimat. Thus, over the course of the 19th century, the Ottoman state became vastly more powerful and organized internally, despite suffering further territorial losses, especially in the Balkans, where a number of new states emerged.The Committee of Union and Progress (CUP) established the Second Constitutional Era in the Young Turk Revolution in 1908, turning the Empire into a constitutional monarchy, which conducted competitive multi-party elections. However, after the disastrous Balkan Wars, the now radicalized and nationalistic CUP took over the government in the 1913 coup d'état, creating a one party regime. The CUP allied the Empire with Germany hoping to escape from the diplomatic isolation which had contributed to its recent territorial losses, and thus joined World War I on the side of the Central Powers. While the Empire was able to largely hold its own during the conflict, it was struggling with internal dissent, especially with the Arab Revolt in its Arabian holdings. During this time, genocide was committed by the Ottoman government against the Armenians, Assyrians, and Greeks. The Empire's defeat and the occupation of part of its territory by the Allied Powers in the aftermath of World War I resulted in its partitioning and the loss of its Middle Eastern territories, which were divided between the United Kingdom and France. The successful Turkish War of Independence, led by Mustafa Kemal Atatürk against the occupying Allies, led to the emergence of the Republic of Turkey in the Anatolian heartland and the abolition of the Ottoman monarchy.