Final answer:
The Battle of Trafalgar in 1805, where the British fleet defeated the French and Spanish fleets, overshadowed Napoleon's land victories and curbed his ability to contest British naval supremacy.
Step-by-step explanation:
The disaster that overshadowed Napoleon's triumph in Vienna was the Battle of Trafalgar in October 1805. While Napoleon was gaining victories on land, the British naval supremacy was cemented at Trafalgar, where a British fleet under Vice Admiral Horatio Nelson defeated the combined French and Spanish fleets.
This pivotal battle ensured that Napoleon could not challenge British control of the sea and ended his ambitions to invade Britain. Subsequently, he shifted his focus to dominating mainland Europe.