The Romulans?
Based on Kirk's line to Spock before firing on Nero's ship:
"Offering compassion might be the only way we can make peace with the Romulans." (or something to that extent)
Unless I missed something - which has been known to happen
- I can only see a few possibilities:
1. Kirk mistakenly thought Nero represented greater Romulan interests. Nero told Pike he didn't represent Romulus, but did this message get through to Kirk?
2. Nero contacted the Romulans in this universe, leading them to declare war with the Federation.
3. The Federation was already at war with the Romulans, or Nero's arrival into this universe altered history to cause that to happen.
4. Following from #1, the Federation is in a cold war with the Romulans (much like the original TOS universe) and "making peace" means detente.
Based on Kirk's line to Spock before firing on Nero's ship:
"Offering compassion might be the only way we can make peace with the Romulans." (or something to that extent)
Unless I missed something - which has been known to happen

1. Kirk mistakenly thought Nero represented greater Romulan interests. Nero told Pike he didn't represent Romulus, but did this message get through to Kirk?
2. Nero contacted the Romulans in this universe, leading them to declare war with the Federation.
3. The Federation was already at war with the Romulans, or Nero's arrival into this universe altered history to cause that to happen.
4. Following from #1, the Federation is in a cold war with the Romulans (much like the original TOS universe) and "making peace" means detente.