Temporal Cold War

The Temporal Cold War was a recurring plot device in Star Trek: Enterprise. Supposedly rival factions from different periods in the Federation's future were attempting to alter history for their own ends, and their efforts often required them to interfere with the course of events involving the Enterprise (NX-01).

The "evil" faction from the future could not easily travel into the past to accomplish their goals, but they could communicate with their 22nd century agents, a faction within the Suliban race. The "good" faction could send its own agents into the past, and at least one secretly joined the crew of the Enterprise for a period of time.

The writers of Enterprise admitted that they didn't really have a plan for how the Temporal Cold War would play out when they created this story arc, and their lack of a plan shows in the nonsensical nature of related story plots.