The Crimean War (1853-1856) resulted from tensions between Russia and France over the protectorship of Christians in the Ottoman Empire, leading to Britain siding with France against Russia. Key battles included the Siege of Sevastopol and the Battle of Sinop, where the allies ultimately defeated Russia and weakened its regional power. The war introduced new military tactics and technologies, alongside significant reforms in military medical practices influenced by figures like Florence Nightingale.