I always liked the "temporal cold war' ideea introduced in star trek: enterprise.
Why?
Because it's plausible, indeed, inevitable in a star trek like universe, where time travel into the past exists.
Much like the real world cold war was all but inevitable in a world with two opposing superpowers armed with nuclear weapons.
Consider:
What would happen if two powers have the ability to travel into the past? Each power would have the ability to destroy the other by changing the past.
And how could you prevent your own destruction? Only by making sure that, should an attack against you undo your past, you will be equally lethal against your opponent - by having temporal agents stationed into the past, ready to strike.
Considering how even for 23rd/24th century powers time travel was a reality, one could speculate that this temporal cold war was already being fought between these powers - its front lines, strategies and tactics taking shape.
Only, of course, all this was happening behind the scenes, much like time travelling technology was still secret.
It's unlikely that 29th century political actors started this temporal cold war.