[Mr. Lewis is a former general director of the Institute of Economic Affairs, London.]

On Oct. 21 1805, off Cape Trafalgar on the Spanish coast, Admiral Lord Nelson won the greatest battle in the annals of sail, routing Napoleon's Navy without losing a single ship himself. The Queen launches this year's bicentennial celebrations in Portsmouth today. It's understandable that the British should honor a triumph that put paid to Napoleon's plans for invading their island, and began a century in which Britannia ruled the waves.

Why should anyone but the Brits commemorate the birth of British imperialism? In addition to the French, Americans look back on that era with misgivings. Trafalgar marked the start of the worst period of Anglo-American relations on record that culminated in an unnecessary war in 1812.

The British blockade of Napoleon's European empire -- in response to the French emperor's closure of Continental ports to British trade -- generated friction between London and Washington. Americans were irked by the Royal Navy's policy of stopping and searching their ships and arresting for desertion any American sailor who had previously served on board British naval vessels. (Indeed when I recently visited Nelson's flagship HMS Victory in Portsmouth, I looked at the list of sailors who served at Trafalgar to see if there was a namesake of mine. There was a Lewis but he was American.) President Thomas Jefferson forbid British ships entry to American waters; Britain, in turn, prohibited all direct trade between America and Europe. Washington declared war just as the British rescinded their Orders in Council prohibiting U.S. trade with Europe. Too late! It took a month for news of this conciliatory gesture to cross the Atlantic. This war proceeded with some notable if small-scale American naval successes, the British burning of the White House and U.S. victories at Baltimore, before peace was restored.

Yet, despite the grievous aftermath, Nelson's victory at Trafalgar was a boon to America as well as to Europe and the rest of the world. America's renowned 19th century naval strategist, Admiral Alfred Thayer Mahan, wrote of the British blockade of France that "those far distant storm-beaten ships, upon which the Grand Army never looked, stood between it and the domination of the world."

Jefferson viewed Napoleon with alarm. When the French colony of Louisiana was ceded to Napoleon by Spain in a secret treaty in 1801, the U.S. president worried that the French had wider designs on the American continent. As Jefferson remarked at the time: "The day that France takes possession of New Orleans, we must marry ourselves to the British fleet and nation." Fortunately Napoleon's troops were turned back from Santo Domingo (today's Haiti), discouraging him from any more adventures. He sold Louisiana -- then about a third of modern America -- for $16 million, or 4 cents an acre.

But Admiral Mahan was still right that had Napoleon beaten Britain and established a European empire, his whole career suggests that he couldn't have stopped there. Napoleon once said: "You can do anything with bayonets except sit on them." America surely would've been next.

As it happens, after Trafalgar, Britannia did rule the waves for a hundred years, not out of benevolence -- except for stamping out the slave trade -- but to protect her commerce. In doing so Britain enabled a huge growth of world trade and prosperity. Moreover, with the Royal Navy standing guard, the danger of Continental European nations interfering in American affairs -- during the Civil War for instance -- was zero. So America was spared the expense of maintaining a large navy for most of the 19th century and instead got on with opening up its west....