Also, the Travelers must follow the six Protocols — 1) the mission comes first; 2) leave the future in the past; 3) don’t take a life, don’t save a life, do not interfere; 4) do not reproduce; 5) maintain your host’s life; and 6) no inter-team communication except in extreme emergencies or when sanctioned.
Travelers can only be sent back as far as the time of the most recent Traveler.
But all of those Protocols are challenged across the series, with varying consequences.
So you can’t just keep going back further and further to revise history, which is good because that would pretty much defeat the urgency of the story.