Napoleon's first defeat in his attempt to take over Europe was

the Peninsula War.

the Battle of Waterloo.

the Battle of Trafalgar.

the Battle of Moscow.

The correct answer is actually:

C. the Battle of Trafalgar

The correct answer is "the Peninsula War".

This first defeat took place in the Battle of Bailen (1808) in the Spanish Southern region of Andalusia, as part of the bigger conflict known as Peninsula War. This was a the military conflict in which the Spanish Kingdom was fighting against the Napoleonic Empire, aiming to get rid of the French occupation in their territory.

The Spanish General Castaños managed to beat the Napoleonic Army for the first time in an open field battle.