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 -- the 28th and 31st centuries -- 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 .
The "evil" 28th-century 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" 31st-century 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 Star Trek: 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 episodes.