The nature of time travel is that it allows a military power with such technology to completely and utterly dominate any foe without access to it. This generally leads to a situation where the first military power to possess time travel technology tends to become the last military power in the history of that universe as well. Usually, this time travelling force proceeds to establish itself at the beginning of time and become the pre-eminent power in the universe to the end of time.
Note however that while only one organization usually achieves temporal superiority in a given universe, parallel universes can often have different organizations achieve temporal superiority. This occasionally leads to the situation where sliders from parallel universes attempt to invade other universes. In practice, this proves very difficult, as the differences between the histories of the two universes generally leads to an advantage for the home team defending against such an invasion.
In the rare event that two civilizations both develop time travel around the same time, before either can achieve temporal superiority, there is often a conflict or temporal war. These grand scale wars generally involve attempts to subvert each other’s plans through increasingly convoluted strategies. Rarely are shots ever actually fired in such wars. Rather, it frequently becomes a matter of who can more effectively outsmart the other in terms of manipulating the causation of events. Eventually, one side gains the upper hand and achieves temporal superiority.
Though, keep in mind that this process often causes multiple branchings of the timeline, and remnants of the losing factions are still present in the earlier segments of the timeline. Similarly, temporal superiority is only established in the final versions of the timeline. Earlier passes often look very different than these.
Thus, any given story can take place in either condition. This explains why there can still be factions competing with each other in past futures, before temporal superiority is properly established. In effect, temporal superiority is only established in future pasts, which leaves the original future untouched, as if time travel never happened.
This bears repeating. Time travel has the effect of branching timelines, such that there is always one old timeline that is unaffected, and one new timeline that is changed. Both exist in parallel. Thus, it is not possible to undo the past, but you can still create an alternate future.