Tropes in time travel stories.
The tiniest effect widens, don't time travel.
The universe branches, you return to a different fork.
.........you can't return...
Or the universe damps out small perturbations.
These affect the minimum intervention to make things better, if we could.
(Nuke Moscow or say "good morning comrade" earlier; put a condom in a drawer Vs assassination", repeated assassinations with large weapons over a century and continents... planets.)