Which country was Napoleon's main adversary throughout the Napoleonic Wars?

The United Kingdom was Napoleon's main adversary throughout the Napoleonic Wars. Britain remained France's most persistent enemy, combining naval power, financial support for continental allies, and direct military intervention.

War resumed between Britain and France in 1803 after the fragile Peace of Amiens collapsed. Napoleon considered invading Britain and gathered troops at Boulogne, but British naval strength made the invasion impossible. The Royal Navy's victory over the combined French and Spanish fleets at Trafalgar in 1805 secured British command of the seas.

Britain also supported resistance to Napoleon on land. In the Peninsular War, British forces under Arthur Wellesley, later the Duke of Wellington, fought alongside Portuguese and Spanish forces against the French. Britain helped fund successive coalitions, including the Seventh Coalition that defeated Napoleon at Waterloo in 1815.

Austria, Prussia, and Russia were formidable opponents, but their wars with Napoleon were intermittent and sometimes followed by alliances or treaties. Britain was the constant strategic rival, which is why it is the canonical answer.