During the War of the Second Coalition (1799–1801), William Pitt the Younger (1759–1806) provided strong leadership in London. Britain occupied most of the French and Dutch overseas possessions, the Netherlands having become a satellite state of France in 1796. After a short peace, in May 1803, war was declared again. Napoleon's plans to invade Britain failed, chiefly due to the inferiority of his navy. In 1805 Lord Nelson's fleet decisively defeated the French and Spanish at Trafalgar, ending any hopes Napoleon had to wrest control of the oceans away from the British.

The British Army remained a minimal threat to France; it maintained a standing strength of just 220,000 men at the height of the Napoleonic Wars, whereas France's armies exceeded a million men—in addition to the armies of numerous allies and several hundred thousand national guardsmen that Napoleon could draft into the French armies when they were needed. Although the Royal Navy effectively disrupted France's extra-continental trade—both by seizing and threatening French shipping and by seizing French colonial possessions—it could do nothing about France's trade with the major continental economies and posed little threat to French territory in Europe. France's population and agricultural capacity far outstripped that of Britain.

In 1806, Napoleon set up the Continental System to end British trade with French-controlled territories. However Britain had great industrial capacity and mastery of the seas. It built up economic strength through trade and the Continental System was largely ineffective. As Napoleon realized that extensive trade was going through Spain and Russia, he invaded those two countries. He tied down his forces in the Peninsular War in Spain, and lost very badly in Russia in 1812. The Spanish uprising in 1808 at last permitted Britain to gain a foothold on the Continent. The Duke of Wellington and his army of British and Portuguese gradually pushed the French out of Spain, and in early 1814, as Napoleon was being driven back in the east by the Prussians, Austrians, and Russians, Wellington invaded southern France. After Napoleon's surrender and exile to the island of Elba, peace appeared to have returned, but when he escaped back into France in 1815, the British and their allies had to fight him again. The armies of Wellington and Blucher defeated Napoleon once and for all at Battle of Waterloo.

Simultaneous with the Napoleonic Wars, trade disputes and British impressment of American sailors led to the War of 1812 with the United States. A central event in American history, it was little noticed in Britain, where all attention was focused on the struggle with France. The British could devote few resources to the conflict until the fall of Napoleon in 1814. American frigates also inflicted a series of embarrassing defeats on the British navy, which was short on manpower due to the conflict in Europe. A full-scale British invasion was defeated in upstate New York. The Treaty of Ghent subsequently ended the war with no territorial changes. It was the last war between Britain and the United States.