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Ten ships of the Franco-Spanish van had been initially left out of the fight during the Battle of Trafalgar. By 2:30 PM, they had successfully maneuvered against the wind and approached the battle. By then, seven ships of the combined fleet had already struck their colors, but had these ten ships reinforced the rest of the combined fleet and counterattacked, it could have turned the tide of the battle around. By that time, ships from both sides were in a bad state, but the combined fleet would now have ten ships in mint condition. A counterattack may well have worked. Instead, they fled southward after a half-hearted exchange of fire.

I don't think that even with a defeat at Trafalgar, Britain would necessarily be doomed. Napoleon's plan called for the French and Spanish fleets in the Mediterranean to break the British blockade and sail for the Carribean, where they would join up, then sail back for France, pierce the British blockade of Brest, and join up with the French fleet trapped in Brest. This massive combined fleet would then clear the English Channel of Royal Navy ships, giving Napoleon's invasion barges safe passage.

Thus, the Royal Navy would still have had plenty of opportunities to put an end to Napoleon's grand scheme. Its navy had almost 1,000 ships (so even if all 33 British ships at Trafalgar were lost, it would be trivial). However, most of this massive fleet was thinly spread around the globe, and as demonstrated by American privateers in the War of 1812, the seas around Britain were not too well-defended. By the time the news hit London, the combined fleet would be well on its way, and it would be useless to try to warn or reinforce the Carribean station. The Royal Navy would probably have reinforced the squadron blockading Brest and recalled ships from around the world to defend the English Channel.
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