The wars grew out of Europe's old dynastic powers treating the new French regime as an existential threat. Napoleon, who seized power by coup in 1799 and crowned himself emperor in 1804, fought a sequence of campaigns against shifting coalitions of Britain, Austria, Russia, and Prussia. Austerlitz in December 1805 — the 'battle of three emperors' — became a textbook case of manoeuvre warfare; the same year, at Trafalgar, the British admiral Nelson annihilated the combined Franco-Spanish fleet. Land supremacy stayed with Napoleon, sea supremacy with Britain.

The wars' strategic engine was a new French army built on the 'levée en masse' — conscription of citizens rather than dynastic regiments. At first this gave overwhelming numerical advantage, but year after year of unbroken campaigning also drained France itself. The Peninsular War of 1807–1812 became a running wound (Goya's 'The Third of May 1808' captured the civilian face of the occupation). In 1812 Napoleon invaded Russia with more than 600,000 men; Borodino was a bloody draw, and taking Moscow proved a strategic trap. Most of the army was destroyed by winter on the retreat.

The 1813 'Battle of the Nations' at Leipzig drove Napoleon out of Germany; in 1814 he was deposed for the first time and exiled to Elba. He returned to France in March 1815 and gathered power for the 'Hundred Days', which ended on 18 June 1815 at Waterloo against the combined armies of Wellington and Blücher. He died in 1821 on the South Atlantic island of Saint Helena. Almost as a side effect of the war, in 1803 he sold the Louisiana territory to the United States — quietly remaking the map of North America.

The Napoleonic Wars were not only military events; they exported law, administration, and measurement. The Napoleonic Code of 1804 became the backbone of modern civil law from the Netherlands through Italy and western Germany, and later across much of Latin America. The metric system spread out of France into the continent. The abolition of feudal privilege, equality before the law, secularization — these were the lasting institutional changes that travelled behind the army. At the same time, the wars stirred the first modern nationalisms in Spain, the German lands, and Russia: the popular resistances against an occupier became the seedbed of nineteenth-century nationalist politics.