Which sentence states a fact?

 - The Vikings came from what are now the countries of Denmark, Sweden, and Norway.
 - The Vikings carried out their raids in a swift, vicious fashion.
The second sentence states a fact.
The Vikings came from what are now the countries of Denmark, Sweden, and Norway.
It can be proved by reading a history book about Vikings.
The first sentence states an opinion.
The Vikings carried out their raids in a swift, vicious fashion.
Vicious shows what a person believes, thinks, or feels. Another person might have a different opinion about what makes a raid vicious.