Conflict with Britain

Unlike its many coalition partners, Britain remained at war throughout the period of the Napoleonic Wars. Protected by naval supremacy (in the words of Admiral Jervis to the House of Lords "I do not say, my Lords, that the French will not come. I say only they will not come by sea"), the United Kingdom maintained low-intensity land warfare on a global scale for over a decade. Despite such assurance by Admiral Jervis, evidence can still be seen of the beacon warning towers built in the event of such an invasion, for example, at Eston Nab, near Middlesbrough. The British Army gave long-term support to the Spanish rebellion in the Peninsular War of 1808–1814. Protected by topography, assisted by massive Spanish guerrilla activity, and sometimes falling back to massive earthworks (The Lines of Torres Vedras), Anglo-Portuguese forces succeeded in harassing French troops for several years. By 1815, the British Army would play the central role in the final defeat of Napoleon at Waterloo.

The Treaty of Amiens (25 March 1802) resulted in peace between the UK and France, but satisfied neither side. Both parties dishonored parts of it: the French intervened in the Swiss civil strife (Stecklikrieg) and occupied several coastal cities in Italy, while the UK occupied Malta. Bonaparte tried to exploit the brief peace at sea to restore the colonial rule in the rebellious Antilles. The expedition, though initially successful, would soon turn to a disaster, with the French commander and Bonaparte’s brother-in-law, Charles Leclerc, dying of yellow fever and almost his entire force destroyed by the disease combined with the fierce attacks by the rebels.

Hostilities between Britain and France renewed on 18 May 1803. The Coalition war-aims changed over the course of the conflict: a general desire to restore the French monarchy became closely linked to the struggle to stop Bonaparte.

Bonaparte declared France an Empire on 18 May 1804 and crowned himself Emperor at Notre-Dame on 2 December.

Having lost most of its colonial empire in the preceding decades, French efforts were focused mainly in Europe. Haiti had won its independence, the Louisiana Territory had been sold to the United States of America, and British naval superiority threatened any potential for France to establish colonies outside Europe. Beyond minor naval actions against British imperial interests, the Napoleonic Wars were much less global in scope than preceding conflicts such as Seven Years' War which historians would term a "world war".

In 1806, Napoleon issued the series of Berlin Decrees, which brought into effect the Continental System. This policy aimed to eliminate the threat from Britain by closing French-controlled territory to its trade. Britain maintained a standing army of just 220,000 at the height of the Napoleonic Wars, whereas France's strength peaked at over 2,500,000, as well as several hundred thousand national guardsmen that Napoleon could draft into the military if necessary; however, British subsidies paid for a large proportion of the soldiers deployed by other coalition powers, peaking at about 450,000 in 1813. The Royal Navy effectively disrupted France's extra-continental trade—both by seizing and threatening French shipping and by seizing French colonial possessions—but could do nothing about France's trade with the major continental economies and posed little threat to French territory in Europe. Also, France's population and agricultural capacity far outstripped that of Britain. However, Britain had the greatest industrial capacity in Europe, and its mastery of the seas allowed it to build up considerable economic strength through trade. That sufficed to ensure that France could never consolidate its control over Europe in peace. However, many in the French government believed that cutting Britain off from the Continent would end its economic influence over Europe and isolate it.