It was in 1945 that World World 2 ended.
I think that is a complementizer, but I'm not sure of the nature of this complement. By nature I mean the part of speech of the complement clause and to what is it a complement.
It's neither the subject nor the object of this sentence. (compare I think that World War 2 ended in 1945.)
It not the complement of a predicate adjective either. (compare I am sure that World War 2 ended in 1945.)
Maybe it's the complement to the prepositional/adverbial phrase in 1945? The reason I say this is because the that-clause couldn't just complement the was if the in 1945 was removed.