S5 - Tensions between the Andorians, Tellerites, Vulcans and humans are mitigated by the need to band against a common enemy. Season finale: Earth-Romulan War begins.
S6 - Entire season devoted to the war. Only Earth is fighting officially but Earth-friendly allies among the Andorians, Tellerites, Vulcans play a role. Lots of Shran and Soval. Unbeknownst to everyone, the Romulans have infiltrated Vulcan society enough to keep them out of the fight through duplicity and double dealing. Season finale: all looks lost for Earth.
S7 - Earth is on the ropes but Archer & the gang manage to cobble together the first incarnation of what will become the Federation as an alliance to save their butts. Season finale: Federation officially founded.
I dunno how long the war should have lasted canon-wise, but remember there's no reason that the rule of one season = one year needs to apply.