The Crimean War was a military conflict that took place in the 1850s between Russia on one side, and the Ottoman Empire, France, and the United Kingdom on the other. It is considered one of the first truly modern wars, with improvements in steamships, artillery, accurate rifle fire, medical care, and other innovations.
The Russian Empire had been expanding at the expense of the Ottoman Empire for some time, and was threatening to take control of the mouth of the Danube River and eventually Constantinople. The United Kingdom and France moved to block this expansion by backing the Ottomans. The Austrians, who had been allied with the Russians since the Napoleonic Wars and more recently the revolutions of 1848, waffled and then broke their alliance, staying neutral while the Russians were attacked.
The British and French launched an invasion of the Crimean peninsula, besieging Sevastapol. Disease killed many of them. The infamous Charge of the Light Brigade, a near suicidal cavalry charge, took place.
The British, French, and Ottomans won, preventing Russia from gaining a direct outlet on the Mediterranean Sea. Austria gained nothing by betraying her ally, and was diplomatically isolated for years afterwards, until being thrashed by Prussia in the 1860s. Austria-Hungary then allied itself with Prussia, in an unequal relationship that eventually allowed them both to be sucked into the disaster known as World War I.