The War of the Third Coalition was a conflict which spanned from 1803 to 1806. It saw the defeat of an alliance of Austria, Portugal, Russia, and others by France and its client states under Napoleon I. Great Britain had already been at war with France following the resumption of hostilities resulting from the breakdown of the Peace of Amiens, and Britain would be the only country still at war with France after the Treaty of Pressburg. For two years (1803–1805) Britain stood under constant threat of a French invasion. The Royal Navy, however, secured mastery of the seas by decisively destroying a Franco-Spanish fleet at the Battle of Trafalgar in October 1805. The Third Coalition itself came to full fruition that summer as Napoleon's provocative actions in Italy (crowning himself King) and Germany  spurred Austria into joining Britain and Russia against France. The war would be decided on the continent, and the major land operations that sealed the swift French victory involved the Ulm Campaign, a large wheeling manoeuvre by the Grande Armée  lasting from late August to mid-October that captured an entire Austrian army, and the decisive French victory over a combined Russo-Austrian force under Tsar Alexander I at the Battle of Austerlitz in early December.