The future as seen in this episode was the prime timeline - the only possible one (until the changes made at the end). It was the result of the Enterprise
-C vanishing from the battle at Narendra Three. Presumably, the Klingons were angry that the Federation (seemingly) walked away from the battle, and that's why the Klingons declared war.
Technically speaking, the 'real' timeline - in which the Ent-C finished the battle and was destroyed - is an alternate timeline, because the ship had to go BACK at the end of the episode.