Why was the Battle of Trafalgar such an important military loss for Napoleon?

The Battle of Trafalgar was an important part of the
Napoleonic Wars.  It was an important military loss for Napoleon because it prevented
him from invading England and because it guaranteed that the British would retain naval
supremacy for the remainder of the war (a major reason they were able to win the
war).

When the Battle of Trafalgar happened in 1805, France
was the strongest military power on the continent of Europe.  By contrast, Britain's
major strength was naval.  If the French could break the naval strength of the British,
the power of their army would likely have been used to invade
England.

At the Battle of Trafalgar, the British destroyed
much of the combined French and Spanish fleets.  By doing so, they kept naval
superiority and made a French invasion of England impossible.