Lie algebras and Lie groups, in particular the very classical linear Lie groups, are strongly connected. I know that some results on reduction or group theory are used to prove results on Lie algebras (Engel reduction theorem for instance). I am wondering if there are some clear examples (maybe elementary or at least classical) of results on Lie algebras, well known or easier to prove, yielding fruits in the domain of group theory?
I have just in mind the classification of classical Lie group, but... For me it is rather a classification of their Lie algebra!