21. This war broke out in the middle of the 19th century. It was fought between Russia and the combined powers of Britain, France, and the Ottoman Empire.
From Quiz Wars and Conflicts of the 19th Century
Answer:
The Crimean War
The Crimean War lasted from 1853 to 1856 and was a military conflict between Russia and a coalition of Great Britain, France, the Kingdom of Sardinia, and the Ottoman Empire. It was a major turning point in the political history of post-Napoleonic Europe. The roots of the conflict lay in the "Eastern Question" resulting from the continuing decay of the Ottoman Empire, a development fraught with explosive implications for the European balance of power.
Since the late 18th century, Russia had become more eager to take advantage of this situation to increase its influence in the Balkans and to wrest from the Ottoman Empire control of the straits between the Black Sea and the Mediterranean Sea. The war was an event of major significance in European history. It marked the collapse of the arrangement whereby the victors of the Napoleonic Wars - Britain, Russia, Austria and Prussia - had cooperated to maintain peace in Europe for four decades. The myth of Russian might was laid to rest, and the breakup of the old coalition permitted Germany and Italy to free themselves from Austrian influence and emerge as nations in the decade that followed. Finally, the shock of the Crimean defeat was the catalyst for a program of sweeping internal reform in Russia under Nicholas's successor, Alexander II.