The Temporal Accord was an interstellar treaty signed after the development of time travel, prohibiting the use of the technology for changing history. The Accord permitted time travel only for the purpose of scientific research, and set forth rules and procedures as to how to do so without contaminating the timeline.
Certain groups who held time travel technology, however, chose to ignore those procedures, and actively attempted to alter history to suit their own ends. These factions were opposed by temporal agents in the 31st century, in a conflict known as the Temporal Cold War. (ENT: "Cold Front", "Shockwave", "Storm Front", "Storm Front, Part II")
The accord was also broken in the episode Timeless, in which an alternate timeline Chakotay and Harry Kim attempt to save Voyager and its crew from a mistake they believe they made.