Star Trek: 10 Things We Now Know About The Temporal Wars
7. Romulan Participation & First Contact
In Tomorrow And Tomorrow And Tomorrow, Sera is revealed as a Romulan agent, one of many sent through time to slow human advancement. The Prime Universe La'An Noonian Singh, along with an alternate version of James T. Kirk, exposes her, though not before she fatally wounds Kirk.
In his version of the timeline, the Romulans have been sending agents to effectively halt any development of the Federation. This would allow them, by the 23rd Century, to launch a massive offensive against the Alpha Quadrant, with the suggestion that they had all but annihilated the Vulcans before La'an's arrival.
It is Sera who reveals that the Romulans' actions shifted Khan's fate around, even though she retains knowledge of the other timeline. She takes a little too much pleasure in 'being the one to kill James T. Kirk', suggesting that the changes have splintered time.
The greater fallout from the Romulans' efforts here delay not only the Eugenics Wars, but also World War III. When Picard and crew, and of course Sisko, Bashir, and Dax, arrive in the early 21st Century, they find cities on the brink, but not yet in the territory, of full-scale dissolution.
As Pike's speech to the Kiley highlights, the wars all tended to blend into one - even if they had begun as individual conflicts. As it stands, first contact with the Vulcans remains a fixed point in history, so it remains to be seen if they arrive at an Earth that is embroiled in a worldwide conflict.