I don’t hate him—he’s just a bore but that isn’t a capital offense. Usually. - Charles Stross

Time travel destabilizes history.
History is a child of contingency; so many events depend on critical misunderstandings or transient encounters that even the apocryphal butterfly’s wing is apt to stir up a storm in short order.

You win some, you lose some. And when you lose, you have to pull yourself together and go back for more. Otherwise, the other side wins by default.

The dirty little secret of the intelligence-gathering job is that information doesn’t just want to be free—it wants to hang out on street corners wearing gang colors and terrorizing the neighbors.