the Napoleonic Wars (1803–1815). During the Napoleonic wars, the Royal Navy victory at Trafalgar (1805) under the command of Horatio Nelson (aboard HMS Victory) marked the culmination of British maritime supremacy, and left the Navy in a position of uncontested hegemony at sea. By 1815 and the conclusion of the Napoleonic Wars, Britain had risen to become the world's dominant great power and the British Empire subsequently presided over a period of relative peace, known as Pax Britannica. With Britain's old rivals no-longer a threat, the nineteenth century saw the emergence of a new rival, the Russian Empire, and a strategic competition in what became known as The Great Game for supremacy in Central Asia. Britain feared that Russian expansionism in the region would eventually threaten the Empire in India. In response, Britain undertook a number of pre-emptive actions against perceived Russian ambitions, including the First Anglo-Afghan War (1839–1842), the Second Anglo-Afghan War (1878–1880) and the British expedition to Tibet (1903–1904). During this period, Britain also sought to maintain the balance of power in Europe, particularly against Russian expansionism, who at the expense of the waning Ottoman Empire had ambitions to "carve up the European part of Turkey". This ultimately led to British involvement in the Crimean War (1854–1856) against the Russian Empire. The beginning of the twentieth century served to reduce tensions between Britain and the Russian Empire, partly due to the emergence of a unified German Empire. The era brought about an Anglo-German naval arms race which encouraged significant advancements in maritime technology (e.g. Dreadnoughts, torpedoes and submarines), and in 1906, Britain had determined that its only likely naval enemy was Germany. The accumulated tensions in European relations finally broke out into the hostilities of the First World War (1914–1918), in what is recognised today, as the most devastating war in British military history, with nearly 800,000 men killed and over 2 million wounded. Allied victory resulted in the defeat of the Central Powers, the end of the German Empire, the Treaty of Versailles and the establishment of the League of Nations. Although Germany had been defeated during the First World War, by 1933 fascism had given rise to Nazi Germany, which under the leadership of Adolf Hitler re-militarised in defiance of the Treaty of Versailles. Once again tensions accumulated in European relations, and following Germany's invasion of Poland in September 1939, the Second World War began (1939–1945).