Second option
Was established after World War II to unify
Option one has a less preferable structure. Putting more clauses in a sentence makes it chunkier and harder to read.
Option three has an inaccurate structure. It has an unnecessary ( or misplaced)full stop mid sentence.
Option four simply changes the subject from EU to World War 2, which is wrong in this context. Seeing this in an article would give someone a nice chuckle)