1
general::
Looking backward, reflecting on the past. For example, In retrospect, he regarded his move as the best thing he'd ever done. This idiom employs retrospect in the sense of "a view of the past." [Second half of 1600s]
American Heritage Idioms
2
general::
Phrase(s): in retrospect [and] in hindsight
reconsidering the past with the knowledge one now has. • In retrospect, I would have gone to a better college. • David realized, in hindsight, that he should have finished school.
McGrawhill's American Idioms And Phrasal Verbs